rtc: cros-ec: add cros-ec-rtc driver.

On platforms with a Chrome OS EC, the EC can function as a simple RTC.
Add a basic driver with this functionality.
Signed-off-by: Stephen Barber <smbarber@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Benson Leung <bleung@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Tested-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/Kconfig

@@ -1255,6 +1255,16 @@ config RTC_DRV_ZYNQMP
 	  If you say yes here you get support for the RTC controller found on
 	  Xilinx Zynq Ultrascale+ MPSoC.
 
+config RTC_DRV_CROS_EC
+	tristate "Chrome OS EC RTC driver"
+	depends on MFD_CROS_EC
+	help
+	  If you say yes here you will get support for the
+	  Chrome OS Embedded Controller's RTC.
+
+	  This driver can also be built as a module. If so, the module
+	  will be called rtc-cros-ec.
+
 comment "on-CPU RTC drivers"
 
 config RTC_DRV_ASM9260

drivers/rtc/Makefile

@@ -44,6 +44,7 @@ obj-$(CONFIG_RTC_DRV_BQ4802)	+= rtc-bq4802.o
 obj-$(CONFIG_RTC_DRV_CMOS)	+= rtc-cmos.o
 obj-$(CONFIG_RTC_DRV_COH901331)	+= rtc-coh901331.o
 obj-$(CONFIG_RTC_DRV_CPCAP)	+= rtc-cpcap.o
+obj-$(CONFIG_RTC_DRV_CROS_EC)	+= rtc-cros-ec.o
 obj-$(CONFIG_RTC_DRV_DA9052)	+= rtc-da9052.o
 obj-$(CONFIG_RTC_DRV_DA9055)	+= rtc-da9055.o
 obj-$(CONFIG_RTC_DRV_DA9063)	+= rtc-da9063.o