iommu/io-pgtable-arm: Avoid warning with 32-bit phys_addr_t

It's not entirely unreasonable for io-pgtable-arm to be built for
configurations with 32-bit phys_addr_t, where the compiler rightly
raises a warning about the 36-bit shift. That particular code path
should never actually *run* on those systems, but we still want it
to compile cleanly, which is easily done by using an unambiguous u64
as the intermediate type instead.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>