irqchip/gic-v3-its: Register the ITS' PA instead of its VA in fwnode

Do not expose the ITS' VA (it appears in debugfs). Instead, record
the PA, which at least can be used to precisely identify the associated
irqchip and domain.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>