irqchip/gic: Register the distributor's PA instead of its VA in fwnode

Do not expose the distributor'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>