irqchip/gic-v4: Add VPE command interface

Add the required interfaces to schedule a VPE and perform a
VINVALL command.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>