omap:usb: add regulator support for EHCI

in case of ehci phy mode; regulator of phy
should be enabled before initializing the
usbhs core driver.

Signed-off-by: Keshava Munegowda <keshava_mgowda@ti.com>
Tested-by: Steve Calfee <stevecalfee@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>