dwc_otg: fix fdt attachment for newer bcm2708-usb nodes

The newer versions of RPi FDT flipped the order of the interrupts
specification and added an 'interrupt-names' property for driver aide in
finding the correct interrupt, rather than assuming the positions. Use it if
it's available, or fallback to the old method if there is no interrupt-names
property with a usb value.

This has been tested with both old RPi3B FDT and new RPi3B FDT, USB again
works on the latter.

Reported by: Tom Yan <tom.ty89 gmail com>
MFC after: 3 days