rtc: add MOXA ART RTC driver

Add RTC driver for MOXA ART SoCs.

Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Reviewed-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>