arm64:mm: initialize max_mapnr using function set_max_mapnr

Initializing max_mapnr using set_max_mapnr() helper function instead
of direct reference. Also not adding PHYS_PFN_OFFSET to max_pfn,
since it already contains it.

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