irqchip/mips-gic: Check the return value of ioremap() in gic_of_init()

The function ioremap() in gic_of_init() can fail, so
its return value should be checked.

Reported-by: Hacash Robot <hacashRobot@santino.com>
Signed-off-by: William Dean <williamsukatube@163.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220723100128.2964304-1-williamsukatube@163.com