clk: mediatek: add CLK_XTAL support for clock driver

This adds the CLK_XTAL macro/flag to allow modeling clocks which are
directly connected to the xtal clock.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>