phy: omap-usb2: Add clock names to Documentation binding

Add "wkupclk" and "refclk" information to DT binding information.

Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>