dev/fdt: Add fdt_foreach_reserved_region

As with fdt_foreach_mem_region add a function that calls a callback
for each reserved memory region in /memreserve. 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/D49699