dm9000: read the mac address from the register if none are specified
as barebox can be run as a second stage the first stage may have init it
before so re-used it
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>