rtc: add support for s35392a

Add support for S35392A RTC. The driver supports both U-Boot driver
models.

Signed-off-by: Nandor Han <nandor.han@ge.com>
Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Cc: Heiko Schocher <hs@denx.de>