phy: phy-mtk-tphy: add a property for disconnect threshold

This is used to tune the threshold of disconnect, the index range
is [0, 15], the threshold voltage is about 400mV for 0, 700mV for
15, and the step is 20mV.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>