ARM: dts: keystone: Add usb phy devicetree bindings

Added device tree support for TI's Keystone USB PHY driver and updated the
Documentation with device tree binding information.

Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>