clk: mediatek: pll: Implement unregister API

The PLL clk type within the MediaTek clk driver library only has a
register function, and no corresponding unregister function. This
means there is no way for its users to properly implement cleanup
and removal.

Add a matching unregister function for the PLL type clk.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Miles Chen <miles.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220208124034.414635-14-wenst@chromium.org


Reviewed-by: Chun-Jie Chen <chun-jie.chen@mediatek.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>