arm64: Add cast for virt_to_pfn

virt_to_pfn lacks a cast at the top level. Don't rely on __virt_to_phys
and explicitly cast to unsigned long.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>