[ARM] vgic: Optimize __vgic_sync_vcpu_hwstate() implementation

This patch does following optimizations to __vgic_sync_vcpu_hwstate():
1. Move maintainence irq related stuff from __vgic_sync_vcpu_hwstate()
to vgic_main_irq().
2. Keep track of used LR register count and skip __vgic_sync_vcpu_hwstate()
when used LR register count is zero.

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