dev/fdt: Add fdt_foreach_reserved_mem

As with fdt_foreach_mem_region add a function that calls a callback
for each reserved memory region. This allows us to exclude this memory
from the physical map in a way that is safe from overflowing a fixed
array on the stack.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D49698