rtc: support for the Amlogic Meson RTC

Add support for the RTC block on the 32-bit Amlogic Meson6, Meson8,
Meson8b and Meson8m2 SoCs.

The RTC is split in to two parts, which are both managed by this driver:
- the AHB front end
- and a simple serial connection to the actual registers

The RTC_COUNTER register which holds the time is 32-bits wide.

There are four 32-bit wide (in total: 16 bytes) "regmem" registers which
are exposed using nvmem. On Amlogic's 3.10 kernel this is used to store
data which needs to survive a suspend / resume cycle.
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[resurrected Ben's patches after 2 years]
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/Kconfig

@@ -1294,6 +1294,17 @@ config RTC_DRV_IMXDI
 	   This driver can also be built as a module, if so, the module
 	   will be called "rtc-imxdi".
 
+config RTC_DRV_MESON
+	tristate "Amlogic Meson RTC"
+	depends on (ARM && ARCH_MESON) || COMPILE_TEST
+	select REGMAP_MMIO
+	help
+	   Support for the RTC block on the Amlogic Meson6, Meson8, Meson8b
+	   and Meson8m2 SoCs.
+
+	   This driver can also be built as a module, if so, the module
+	   will be called "rtc-meson".
+
 config RTC_DRV_OMAP
 	tristate "TI OMAP Real Time Clock"
 	depends on ARCH_OMAP || ARCH_DAVINCI || COMPILE_TEST

drivers/rtc/Makefile

@@ -101,6 +101,7 @@ obj-$(CONFIG_RTC_DRV_MAX8997)	+= rtc-max8997.o
 obj-$(CONFIG_RTC_DRV_MAX8998)	+= rtc-max8998.o
 obj-$(CONFIG_RTC_DRV_MC13XXX)	+= rtc-mc13xxx.o
 obj-$(CONFIG_RTC_DRV_MCP795)	+= rtc-mcp795.o
+obj-$(CONFIG_RTC_DRV_MESON)	+= rtc-meson.o
 obj-$(CONFIG_RTC_DRV_MOXART)	+= rtc-moxart.o
 obj-$(CONFIG_RTC_DRV_MPC5121)	+= rtc-mpc5121.o
 obj-$(CONFIG_RTC_DRV_MSM6242)	+= rtc-msm6242.o