MIPS: BCM63XX: provide a MAC address for BCM3368 chips

The BCM3368 SoC uses a NVRAM format which is not compatible with the one
used by CFE, provide a default MAC address which is suitable for use and
which is the default one also being used by the bootloader on these
chips.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: cernekee@gmail.com
Cc: jogo@openwrt.org
Patchwork: https://patchwork.linux-mips.org/patch/5498/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>