ramips: Add support for ZBT WG3526

The WG3526 is the follow-up to the 2626 and is mostly the same, with the
excaption that the mt7602 has been replaced with the mt7603. The internal wifi
setup has also changed slightly. Based on my tests, everything that worked on
the 2626 works on the 3526 and with roughly the same performance.

v1->v2:
* Remove some references to 2626 that I had missed in the dts.

v2->v3:
* Update patch to match new file structure.
* Removed SD driver to be consistent with other MT7621 targets.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>

SVN-Revision: 49213