usb: xhci-mtk: add an optional xhci_ck clock

Some SoCs may have an optional clock xhci_ck (125M or 200M), it
usually uses the same PLL as sys_ck, so support it.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/1566542425-20082-2-git-send-email-chunfeng.yun@mediatek.com


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