hw/intc/arm_gic: reserved register addresses are RAZ/WI

The GICv2 specification says that reserved register addresses
must RAZ/WI; now that we implement external abort handling
for Arm CPUs this means we must return MEMTX_OK rather than
MEMTX_ERROR, to avoid generating a spurious guest data abort.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1513183941-24300-3-git-send-email-peter.maydell@linaro.org
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>