The composite clock didn't have any unregistration function, which forced
us to use clk_unregister directly on it.

While it was already not great from an API point of view, it also meant
that we were leaking the clk_composite structure allocated in
clk_register_composite.

Add a clk_unregister_composite function to fix this.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>