rtc: ac100: support clock-output-names in device tree binding

rtc: ac100: support clock-output-names in device tree binding

The ac100 device tree binding specifies the usage of clock-output-names
to specify the names of its 3 clock outputs. This is needed for orphan
clock resolution, when the ac100 is probed much later than any clocks
that consume any of its outputs. This wasn't supported by the driver.

Add support for this.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>