usb: phy: omap-usb2: enable 960Mhz clock for omap5

"usb_otg_ss_refclk960m" is needed for usb2 phy present in omap5. For
omap4, the clk_get of this clock will fail since it does not have this
clock.

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