irqchip/gic-v2m: Register the frame's PA instead of its VA in fwnode

Do not expose the frame's 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>