phy: phy-mtk-tphy: add a new reference clock

Usually the digital and analog phys use the same reference clock,
but some platforms have two separate reference clocks for each of
them, so add another optional clock to support them.
In order to keep the clock names consistent with PHY IP's, change
the da_ref for analog phy and ref clock for digital phy.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Weijie Gao <weijie.gao@mediatek.com>