rtc: snvs: select option REGMAP_MMIO

Select REGMAP_MMIO to fix the following build error seen with x86_64
randconfig.

  drivers/built-in.o: In function `snvs_rtc_probe':
rtc-snvs.c:(.text+0x567730): undefined reference to `devm_regmap_init_mmio_clk'
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>
Acked-by: NFrank Li <Frank.Li@freescale.com>
Acked-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/Kconfig

@@ -1523,6 +1523,7 @@ config RTC_DRV_MXC
 
 config RTC_DRV_SNVS
 	tristate "Freescale SNVS RTC support"
+	select REGMAP_MMIO
 	depends on HAS_IOMEM
 	depends on OF
 	help