irqchip/gic-v3-its: Fix access width for gicr_syncr

GICR_SYNCR is a 32bit register, so it is better to access it with
32bit access width, though we have not seen any real problem.

Signed-off-by: Heyi Guo <guoheyi@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200225090023.28020-1-guoheyi@huawei.com