rtc: Add rtc driver for the Loongson family chips

The Loongson family chips use an on-chip counter 0 (Time Of Year
counter) as the RTC. We will refer to them as rtc-loongson.

Cc: Keguang Zhang <keguang.zhang@gmail.com>
Cc: Yang Ling <gnaygnil@gmail.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@kernel.org>
Signed-off-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Keguang Zhang <keguang.zhang@gmail.com>
Tested-by: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com> # LS7A
Link: https://lore.kernel.org/r/0c5171156390f614d72f36ceb04a20f432ca639e.1685693501.git.zhoubinbin@loongson.cnSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/Kconfig

@@ -1687,6 +1687,19 @@ config RTC_DRV_JZ4740
 	  This driver can also be built as a module. If so, the module
 	  will be called rtc-jz4740.
 
+config RTC_DRV_LOONGSON
+	tristate "Loongson On-chip RTC"
+	depends on MACH_LOONGSON32 || MACH_LOONGSON64 || COMPILE_TEST
+	select REGMAP_MMIO
+	help
+	  This is a driver for the Loongson on-chip Counter0 (Time-Of-Year
+	  counter) to be used as a RTC.
+	  It can be found on Loongson-1 series cpu, Loongson-2K series cpu
+	  and Loongson LS7A bridge chips.
+
+	  This driver can also be built as a module. If so, the module
+	  will be called rtc-loongson.
+
 config RTC_DRV_LPC24XX
 	tristate "NXP RTC for LPC178x/18xx/408x/43xx"
 	depends on ARCH_LPC18XX || COMPILE_TEST

drivers/rtc/Makefile

@@ -78,6 +78,7 @@ obj-$(CONFIG_RTC_DRV_ISL12022)	+= rtc-isl12022.o
 obj-$(CONFIG_RTC_DRV_ISL12026)	+= rtc-isl12026.o
 obj-$(CONFIG_RTC_DRV_ISL1208)	+= rtc-isl1208.o
 obj-$(CONFIG_RTC_DRV_JZ4740)	+= rtc-jz4740.o
+obj-$(CONFIG_RTC_DRV_LOONGSON)	+= rtc-loongson.o
 obj-$(CONFIG_RTC_DRV_LP8788)	+= rtc-lp8788.o
 obj-$(CONFIG_RTC_DRV_LPC24XX)	+= rtc-lpc24xx.o
 obj-$(CONFIG_RTC_DRV_LPC32XX)	+= rtc-lpc32xx.o