Make the memory size returned from fdt_get_mem_regions a 64-bit type. This
is the physical memory size so may be larger than a u_long can hold, e.g.
on ARM with LPAE we could see an address space of up to 40 bits. On ARM
u_long is only 32 bits so the memory size will be truncated, possibly to
zero.

Reported by: bz
Sponsored by: ABT Systems Ltd