phy: dt bindings: add NXP LPC18xx/43xx USB OTG PHY bindings

Add binding documentation for NXP LPC18xx/43xx USB OTG PHY. This
PHY can found on NXP LPC18xx and LPC43xx devices with USB support.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>