arm64: mm: use phys_addr_t instead of unsigned long in __map_memblock

Cosmetic change to use phys_addr_t instead of unsigned long for the
return value of __pa_symbol().

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>