serial: mtk: add support for using dynamic baud clock souce

serial: mtk: add support for using dynamic baud clock souce

The baud clock on some platform may change due to assigned-clock-parent
set in DT. In current flow the baud clock is only retrieved during probe
stage. If the parent of the source clock changes after probe stage, the
setbrg will set wrong baudrate.

To get the right clock rate, this patch records the baud clk struct to the
driver's priv, and changes the driver's flow to get the clock rate before
calling _mtk_serial_setbrg().

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