ARM: OMAP2+: Provide alias to USB PHY clock

Till the OMAP clocks are correctly defined in device tree, use
this temporary hack to provide clock alias to the USB PHY clocks.

Without this, USB Host & Ethernet will not be functional with
device tree boots on Panda and uEVM.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>