arm64: Add a helper for PARange to physical shift conversion

On arm64, ID_AA64MMFR0_EL1.PARange encodes the maximum Physical
Address range supported by the CPU. Add a helper to decode this
to actual physical shift. If we hit an unallocated value, return
the maximum range supported by the kernel.
This will be used by KVM to set the VTCR_EL2.T0SZ, as it
is about to move its place. Having this helper keeps the code
movement cleaner.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Christoffer Dall <cdall@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>