[POWERPC] bootwrapper: Factor out dt_set_mac_address()

This allows callers to set addresses one at a time when that would be more
convenient.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>