ARM: dts: sun9i: Switch to the AC100 RTC clock outputs for osc32k

The 32.768 kHz clock inside the A80 SoC is fed from an external source,
typically the AC100 RTC module.

Make the osc32k placeholder a fixed-factor clock so board dts files can
specify its source.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>