arm64: mm: update max pa bits to 48

Now that we support 48-bit physical addressing, update MAX_PHYSMEM_BITS
accordingly.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@caviumnetworks.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>