[ARM] vgic: Minor improvement to __vgic_xxx() functions

The __vgic_queue_irq() and __vgic_sync_hwstate_vcpu() are very
crucial part of VGIC emulation and called quiet frequently
hence this patch marks few local variables of these function
to be registers.

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