Allow callers of OF_decode_addr to get the size of the found mapping. This
will allow for code that uses the old fdt_get_range and fdt_regsize
functions to find a range, map it, access, then unmap to replace this, up
to and including the map, with a call to OF_decode_addr.

As this function should only be used in the early boot code the unmap is
mostly do document we no longer need the mapping as it's a no-op, at least
on arm.

Reviewed by: jhibbits
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D5258