usb: ehci-omap: Add OMAP4 support

Update the ehci-omap glue layer to support the controller in the
OMAP4. Major differences from OMAP3 is that the OMAP4 has per-port
clocking, and supports ULPI output clocking mode. The old input
clocking mode is not supported.

Also, there are only 2 externally available ports as against 3
in the OMAP3. The third port is internally tied off and should
not be used.

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