usb: xhci-mtk: use clock bulk to get clocks

Use clock bulk helpers to get/enable/disable clocks, meanwhile
make sys_ck optional, then will be easier to handle clocks.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1618031406-15347-4-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>