ARM: at91: PM: add cpu idle support for sama7g5

Add CPU idle support for SAMA7G5. Support will make use of PMC_CPU_RATIO
register to divide the CPU clock by 16 before switching it to idle and
use automatic self-refresh option of DDR controller.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220113144900.906370-5-claudiu.beznea@microchip.com

include/linux/clk/at91_pmc.h

@@ -78,6 +78,10 @@
 #define		AT91_PMC_MAINRDY	(1	<< 16)		/* Main Clock Ready */
 
 #define	AT91_CKGR_PLLAR		0x28			/* PLL A Register */
+
+#define	AT91_PMC_RATIO		0x2c			/* Processor clock ratio register [SAMA7G5 only] */
+#define		AT91_PMC_RATIO_RATIO	(0xf)		/* CPU clock ratio. */
+
 #define	AT91_CKGR_PLLBR		0x2c			/* PLL B Register */
 #define		AT91_PMC_DIV		(0xff  <<  0)		/* Divider */
 #define		AT91_PMC_PLLCOUNT	(0x3f  <<  8)		/* PLL Counter */