clock-generic: Use separate list for recursively setting parents

Previous implementation of recursive set parent iterates through all
clocks in mux source list, trying to find the correct parent. This is not
desirable because we only want to recurse on a subset of mux sources.

Split mux source list into parents and rec_parents. Only for sources in
rec_parents, mux_set_parent() will try to recursively search their parent.

Change-Id: I041ac4018105db3459b2135446b948706ead3cae
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>