phy: phy-mtk-tphy: add a property for internal resistance

This is used to tune J-K voltage by internal R (resistance), the
range is [0, 31], the resistance value is about 6.9K ohm for 0,
3.8K ohm for 31, and the step is 1K ohm

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