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

Usually the digital and analog phys use the same reference clock,
but on some platforms, they are separated, so add another optional
clock to support it.
In order to keep the clock names consistent with PHY IP's, use
the da_ref for analog phy and ref clock for digital phy.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>