ar71xx: Add support for wdr7500v3

The device is similar to Archer C7 v2, but with 8M of flash, and 6 external
antennas.  Reason for breaking the naming pattern is that sticker on the back
mentions "WDR7500" and has no mention of "Archer".

Signed-off-by: Srdjan Rosic <rosic@google.com>

SVN-Revision: 47344