realtek: clock driver: allow multiple clock parents

realtek: clock driver: allow multiple clock parents

RTL930X LXB clock is not derived from the crystal. In a future commit
we will add that and need the possibility to derive a clock from something
else than the crystal. Make this configurable per SOC type.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>