ramips: add TEW-691GR/TEW-692GR 2.4 GHz interface mac addresses

ramips: add TEW-691GR/TEW-692GR 2.4 GHz interface mac addresses

Assign an unused MAC addresses to the 2.4GHz wifi interface as it was
originally planed but not possible.

The MAC address numbering of the TEW-691GR changes to the following
pattern:

  LAN:  AA:BB:CC:DD:EE:0C
  WIFI: AA:BB:CC:DD:EE:0D
  WAN:  AA:BB:CC:DD:EE:0F

The MAC address numbering of the TEW-692GR changes to the following
pattern:

  LAN:    AA:BB:CC:DD:EE:e4
  WAN:    AA:BB:CC:DD:EE:e5
  2.4GHz: AA:BB:CC:DD:EE:e7
  5 GHz:  AA:BB:CC:DD:EE:e8

Set the label and compatible string for the TEW-692GR PCIe wireless
according to the the PCI binding documentation.

Use the wifi@0,0 label and the pci0,0 compatible string since the PCI
vendor and device id is unknown. It should work anyway since the
compatible string isn't evaluated (yet).

Signed-off-by: Mathias Kresin <dev@kresin.me>