[EMULATORS] pic: Fix usage of GIC_BASE_IRQ().

The GIC_BASE_IRQ() or base_irq field of GIC state is for nested GICs
so, its use in __gic_dist_readb() and __gic_dis_writeb() is wrong and
will not work for child GICs.

This patch fixes this use of GIC_BASE_IRQ().

Signed-off-by: Anup Patel <anup@brainfault.org>