[POWERPC] Convert to mac-address for ethernet MAC address data.

Also accept "local-mac-address".  However the old "address"
is now obsolete, but accepted for backwards compatibility.
It should be removed after all device trees have been
converted to use "mac-address".

Signed-off-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>