arm64: Make the PHYS_MASK_SHIFT dynamic

Make the PHYS_MASK_SHIFT dynamic for Realms. This is only is required
for masking the PFN from a pte entry. Elsewhere, we could still use the
PA bits configured by the kernel. So, this patch:

 -> renames PHYS_MASK_SHIFT -> MAX_PHYS_SHIFT as supported by the kernel
 -> Makes PHYS_MASK_SHIFT -> Dynamic value of the (I)PA bit width
 -> For a realm: reduces phys_mask_shift if the RMM reports a smaller
    configured size for the guest.

Co-developed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>