arm: Make physical address mask dynamic

We are about to add Realm support where the physical address width may be known
via RSI. Make the Physical Address mask dynamic, so that it can be adjusted
to the limit for the realm. This will be required for making pages shared, as
we introduce the "sharing" attribute as the top bit of the IPA.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>