arm: omap: usb: usbhs core device initialization

A new usbhs platform device is defined;
this device will be the parent device of ehci and
ohci platform devices. the usbhs_init function
is defined which does the usbhs device initialization
and I/O mux of ehci and ohci.

Signed-off-by: Keshava Munegowda <keshava_mgowda@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>