phy: omap-usb2: Use generic clock names "wkupclk" and "refclk"

As clocks might be named differently on multiple platforms, use a generic
name in the driver and allow device tree node to specify the platform
specific clock name.

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>