BCM270X_DT: usb: Refactor DTS and overlays

Move the IRQ interrupt declaration in the usb node before the FIQ
declaration, so that the dwc2 driver will find it. Name the
interrupts appropriately so that the dwc_otg driver can still find
them. Then remove the interrupt rewriting from the overlays.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>