msm: clock: Allow set_parent on clocks without the set_parent op

Some use cases require that set_parent succeed on a clock
that actually has just one parent. If a clock doesn't expose
a set_parent op, but the requested parent is the same as the
current parent, return success.

Change-Id: I0cf3f1e6e982de5cc86eb408ca71a62466710ef1
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>