rtc: ac100: Add RTC driver for X-Powers AC100

X-Powers AC100 is a codec / RTC combo chip. This driver supports
the RTC sub-device.

The RTC block also has clock outputs and non-volatile storage.
Non-volatile storage wthin the RTC hardware is not supported.
Clock output support is added in the next patch.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>