utilfdt_read: pass back up the length of data read

For a follow up commit, we want to be able to scan the buffer that was
returned to us.  In order to do that safely, we need to know how big
the buffer actually is, so create a new set of funcs to pass that back.

Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>