[DRIVERS] irqchip: Reduce dependency of GIC driver on irq_start parameter

Currently, the GIC irqchip driver depends more on irq_start parameter
passed via device tree node. This irq_start parameter/attribute is
non-standard and can be assumed based on GIC instance is primary
or secondary. For example, if GIC is primary interrupt controller
then irq_start can be safely assumed to be 16 whereas for secondary
GIC irq_start can be assumed to be 32.

This patch simplifies GIC irqchip driver to reduce dependency on the
irq_start parameter/attribute.

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