hw/intc/arm_gicv3: Make reserved register addresses RAZ/WI

The GICv3 specification says that reserved register addresses
should RAZ/WI. This means we need to return MEMTX_OK, not MEMTX_ERROR,
because now that we support generating external aborts the
latter will cause an abort on new board models.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1513183941-24300-2-git-send-email-peter.maydell@linaro.org
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>