clock-generic: Expose parent_set_rate for code reuse

Pass clk_set_rate() onto parent is a behavior that some clk_ops
might want besides external clock types. Expose it for better
code reuse.

Change-Id: Iae4d20fc23003f14f19b4a93ec598096ef088967
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>