usb: ehci: introduce CONFIG_USB_EHCI_HCD_OMAP

Introduce the CONFIG_USB_EHCI_HCD_OMAP option to select
EHCI support on OMAP3 and later chips. This scales better
than having a long line of dependencies for each new OMAP
with EHCI support.

Signed-off-by: Keshava Munegowda <keshava_mgowda@ti.com>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>