irqchip/gic-v3-its: Add VPE irq domain [de]activation

On activation, a VPE is mapped using the VMAPP command, followed
by a VINVALL for a good measure. On deactivation, the VPE is
simply unmapped.

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