[EMULATORS] pic: Few cosmetic changes for GIC emulator

This patch replaces few 'int' fields in GIC state with 'u32'
to ensure all arithmetic operations are unsigned.

It also adds GIC_BASE_IRQ() macro to access base_irq from
given GIC state thereby making it similar as num_irq and
num_cpu of GIC state.

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