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: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Acked-by: Frank Li <Frank.Li@freescale.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>