Fix fdt_get_mem_regions() to work with 64-bit addresses

Use u_long instead of uint32_t variables to avoid overflow
in case of PA space bigger than 32-bit.

Obtained from: Semihalf
Submitted by: Michal Stanek <mst@semihalf.com>
Sponsored by: Annapurna Labs
Approved by: cognet (mentor)
Reviewed by: andrew, br, wma
Differential revision: https://reviews.freebsd.org/D5393