rtc: snvs: use syscon to access register

snvs included rtc, on/off key, power-off module
change to syscon to access register

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>