irqchip/gic-v3-its: Print decoding of GITS_IIDR when probing for quirks

Similar to the distributor, it is probably a good idea to call out the
details of a given ITS instance if someone is debugging GIC related
issues. Decode the register and print it when probing for quirks on the
ITS.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>