soc: sam0: Fix SAMD20 IRQ assignments

SAMD20 does not have the DMA or USB peripherals and as a result
the IRQs for all subsequent lines are shifted down from SAMD21.
This splits the interrupt assignment for the SERCOMs into the
SoC specific DTS file and moves the USB definition to SAMD21 only.

Signed-off-by: Derek Hageman <hageman@inthat.cloud>