ethernet: ucc_geth: factor out parsing of {rx,tx}-clock{,-name} properties

Reduce the code duplication a bit by moving the parsing of
rx-clock-name and the fallback handling to a helper function.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>