rt2x00: add support for mac addr from device tree

On some devices the EEPROMs of Ralink Wi-Fi chips have a default Ralink
MAC address set (RT3062F: 00:0C:43:30:62:00, RT3060F:
00:0C:43:30:60:00). Using multiple of these devices in the same network
can cause nasty issues.

Allow to override the MAC in the EEPROM with (a known good) one set in
the device tree to bypass the issue.

Signed-off-by: Mathias Kresin <dev@kresin.me>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>