hwmon: Add sophgo SG2042 external hardware monitor support

SG2042 use an external MCU to provide basic hardware information
and thermal sensors.

Add driver support for the onboard MCU of SG2042.

Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Message-ID: <IA1PR20MB49536C786048D1E676BB9C20BB822@IA1PR20MB4953.namprd20.prod.outlook.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/index.rst

@@ -206,6 +206,7 @@ Hardware Monitoring Kernel Drivers
    sch5636
    scpi-hwmon
    sfctemp
+   sg2042-mcu
    sht15
    sht21
    sht3x

Documentation/hwmon/sg2042-mcu.rst

@@ -0,0 +1,78 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Kernel driver sg2042-mcu
+========================
+
+Supported chips:
+
+  * Onboard MCU for sg2042
+
+    Addresses scanned: -
+
+    Prefix: 'sg2042-mcu'
+
+Authors:
+
+  - Inochi Amaoto <inochiama@outlook.com>
+
+Description
+-----------
+
+This driver supprts hardware monitoring for onboard MCU with
+i2c interface.
+
+Usage Notes
+-----------
+
+This driver does not auto-detect devices. You will have to instantiate
+the devices explicitly.
+Please see Documentation/i2c/instantiating-devices.rst for details.
+
+Sysfs Attributes
+----------------
+
+The following table shows the standard entries support by the driver:
+
+================= =====================================================
+Name              Description
+================= =====================================================
+temp1_input       Measured temperature of SoC
+temp1_crit        Critical high temperature
+temp1_crit_hyst   hysteresis temperature restore from Critical
+temp2_input       Measured temperature of the base board
+================= =====================================================
+
+The following table shows the extra entries support by the driver
+(the MCU device is in i2c subsystem):
+
+================= ======= =============================================
+Name              Perm    Description
+================= ======= =============================================
+reset_count       RO      Reset count of the SoC
+uptime            RO      Seconds after the MCU is powered
+reset_reason      RO      Reset reason for the last reset
+repower_policy    RW      Execution policy when triggering repower
+================= ======= =============================================
+
+``repower_policy``
+  The repower is triggered when the temperature of the SoC falls below
+  the hysteresis temperature after triggering a shutdown due to
+  reaching the critical temperature.
+  The valid values for this entry are "repower" and "keep". "keep" will
+  leave the SoC down when the triggering repower, and "repower" will
+  boot the SoC.
+
+Debugfs Interfaces
+------------------
+
+If debugfs is available, this driver exposes some hardware specific
+data in ``/sys/kernel/debug/sg2042-mcu/*/``.
+
+================= ======= =============================================
+Name              Format  Description
+================= ======= =============================================
+firmware_version  0x%02x  firmware version of the MCU
+pcb_version       0x%02x  version number of the base board
+board_type        0x%02x  identifiers for the base board
+mcu_type          %d      type of the MCU: 0 is STM32, 1 is GD32
+================= ======= =============================================