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>