arm: GIC: handle distributor accesses

arm: GIC: handle distributor accesses

This patch adds the handling of MMIO accesses to the GICv3 distributor.
By restricting the SPI masks to the cell's configuration, it makes sure
that they do not touch the other cell's SPI's when writing the common
registers.
Except for the routing and SGIR registers, most of the code should be
common to both GICv2 and v3.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
[Jan: switch to mmio accessor]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>