irqchip/gic-v2m: Add const to of_device_id

struct of_device_id should normally be const.

Signed-off-by: Xiang wangx <wangxiang@cdjrlc.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211209132453.25623-1-wangxiang@cdjrlc.com