arm64: ARCH_PFN_OFFSET should be unsigned long

pfns are unsigned long, but PHYS_PFN_OFFSET is phys_addr_t. This leads
to page_to_pfn() returning phys_addr_t which cause type mismatches in
some print statements.

Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>