phy: add lpc18xx usb otg phy driver

Add PHY driver for the internal USB OTG PHY found on NXP
LPC18xx and LPC43xx devices. This driver takes care of
enabling the PHY in CREG (syscon) and setting the required
clock frequency.

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