serial: 8250-mtk: modify mtk uart power and clock management

MTK uart design no need to control uart clock,
so we just control bus clock in runtime function.
Add uart clock used count to avoid repeatedly switching the clock.

Signed-off-by: Changqi Hu <changqi.hu@mediatek.com>
Link: https://lore.kernel.org/r/1582707225-26815-1-git-send-email-changqi.hu@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>