clk: sunxi: factors: Add unregister function

sunxi's factors clk did not have an unregister function. This means
multiple structs were leaked whenever a factors clk was unregistered.

Add an unregister function for it. Also keep pointers to the mux and
gate structs so they can be freed.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>