dev-arm: Get a Gicv3Redistributor ptr from phys address

dev-arm: Get a Gicv3Redistributor ptr from phys address

The patch is adding the following method to Gicv3:

* Gicv3::getRedistributorByAddr
This will be needed by the ITS when trying to select the target
redistributor after decoding the collection table entry (RDBase).

Change-Id: I40e2c155f2fdc8ca6d3c20ff7a27702e02499f20
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18597
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>