ARM: add 32bit support to GICv3

Implement the system and memory-mapped register accesses in
asm/arch_gicv3.h for 32bit architectures.

This patch is a straightforward translation of the arm64 header. 64bit
accesses are done in two times and don't need atomicity: TYPER is
read-only, and the upper-word of IROUTER is always zero on 32bit
architectures.

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>