ramips: add label MAC address for Xiaomi MIR3G

ramips: add label MAC address for Xiaomi MIR3G

The device label shows the address currently assigned to the OpenWrt
LAN interface.

Current setup is:
LAN  *:b8  factory 0xe006  label
WAN  *:b7  factory 0xe000

For vendor FW bootlog we get (manually removed parts of the address):
[    7.520000] set LAN/WAN LWLLL
[    7.530000] GMAC1_MAC_ADRH -- : 0x00004031
[    7.530000] GMAC1_MAC_ADRL -- : 0x3c****b7
[    7.530000] GDMA2_MAC_ADRH -- : 0x00004031
[    7.540000] GDMA2_MAC_ADRL -- : 0x3c****b8
[    7.540000] eth1: ===> VirtualIF_open

Without further information, this does not allow verification of
the currently unexpected LAN/WAN assignment (we would expect 0xe000
to be LAN).

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>