usb: dwc3: omap: minor fixes to get dt working

Includes few minor fixes in dwc3-omap like populating the compatible
string in a correct way, extracting the utmi-mode property properly and
changing the index of get_irq since irq of core is removed from hwmod
entry.

Also updated the documentation with dwc3-omap device tree binding
information.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
[ balbi@ti.com : fix a compile warning introduced by
	this commit ]

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