rtc: sun6i: Add sun6i RTC driver

This patch introduces the driver for the RTC in the Allwinner A31 and
A23 SoCs.

Unlike the RTC found in A10/A20 SoCs, which was part of the timer, the
RTC in A31/A23 are a separate hardware block, which also contain a few
controls for the RTC block hardware (a regulator and RTC block GPIO pin
latches), while also having separate interrupts for the alarms.

The hardware is different enough to make a different driver for it.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Varka Bhadram <varkabhadram@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

Documentation/devicetree/bindings/rtc/sun6i-rtc.txt

+* sun6i Real Time Clock
+
+RTC controller for the Allwinner A31
+
+Required properties:
+- compatible	: Should be "allwinner,sun6i-a31-rtc"
+- reg		: physical base address of the controller and length of
+		  memory mapped region.
+- interrupts	: IRQ lines for the RTC alarm 0 and alarm 1, in that order.
+
+Example:
+
+rtc: rtc@01f00000 {
+	compatible = "allwinner,sun6i-a31-rtc";
+	reg = <0x01f00000 0x54>;
+	interrupts = <0 40 4>, <0 41 4>;
+};

drivers/rtc/Kconfig

@@ -1175,6 +1175,13 @@ config RTC_DRV_SUN4V
 	  If you say Y here you will get support for the Hypervisor
 	  based RTC on SUN4V systems.
 
+config RTC_DRV_SUN6I
+	tristate "Allwinner A31 RTC"
+	depends on MACH_SUN6I || MACH_SUN8I
+	help
+	  If you say Y here you will get support for the RTC found on
+	  Allwinner A31.
+
 config RTC_DRV_SUNXI
 	tristate "Allwinner sun4i/sun7i RTC"
 	depends on ARCH_SUNXI

drivers/rtc/Makefile

@@ -128,6 +128,7 @@ obj-$(CONFIG_RTC_DRV_STARFIRE)	+= rtc-starfire.o
 obj-$(CONFIG_RTC_DRV_STK17TA8)	+= rtc-stk17ta8.o
 obj-$(CONFIG_RTC_DRV_STMP)	+= rtc-stmp3xxx.o
 obj-$(CONFIG_RTC_DRV_SUN4V)	+= rtc-sun4v.o
+obj-$(CONFIG_RTC_DRV_SUN6I)	+= rtc-sun6i.o
 obj-$(CONFIG_RTC_DRV_SUNXI)	+= rtc-sunxi.o
 obj-$(CONFIG_RTC_DRV_TEGRA)	+= rtc-tegra.o
 obj-$(CONFIG_RTC_DRV_TEST)	+= rtc-test.o