MAINTAINERS: Add myself as the ARM GIC maintainer

The ARM GIC maintenance is currently covered by the blanket
IRQCHIP DRIVERS entry, which I'm about to remove myself from.

It is unlikely that anyone is mad enough to pick this up,
so I'll keep doing that for the foreseable future.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Zenghui Yu <zenghui.yu@linux.dev>
Link: https://lore.kernel.org/r/20231002141302.3409485-2-maz@kernel.org