clk: mux: Add unregistration API

Some drivers want to remove clocks they register with
clk_unregister(). Unfortunately, calling clk_unregister()
directly on the clock returned by clk_register_mux() would result
in a memory leak of the clk_mux struct. Add an API to free that
allocation and unregister the clock.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>