drivers/rtc/rtc-pcf2123.c: add support for devicetree

Add compatible string "nxp,rtc-pcf2123"
Document the binding

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/devicetree/bindings/rtc/nxp,rtc-2123.txt

@@ -0,0 +1,16 @@
+NXP PCF2123 SPI Real Time Clock
+
+Required properties:
+- compatible: should be: "nxp,rtc-pcf2123"
+- reg: should be the SPI slave chipselect address
+
+Optional properties:
+- spi-cs-high: PCF2123 needs chipselect high
+
+Example:
+
+rtc: nxp,rtc-pcf2123@3 {
+	compatible = "nxp,rtc-pcf2123"
+	reg = <3>
+	spi-cs-high;
+};