[ARM] pxa/colibri: provide MAC address from ATAG_SERIAL

In 67fca028

, the ax88796 ethernet driver
learned a way to let the platform data hand in the MAC address. Use it
here as the original Colibri bootloader passes in a MAC address via
ATAG_SERIAL.

Reported-by: Matthias Meier <matthias.j.meier@gmx.net>
Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Eric Miao <eric.miao@marvell.com>