ramips: Improve Sanlinking D240 config

ramips: Improve Sanlinking D240 config

* The left most mini-PCIe slot (the one attached to SIM2) can be
power-cycled by setting GPIO 0 to high/low.

* The D240 only needs the MT76x2 module, so update makefile to reflect this.
Note that until the default mt7620 target is updated, then kmod-mt76 (and thus
kmod-mt7603) will be selected by default.

v2->v3:
* Indentation error.

v1->v2:
* Rename gpio and remove redundant comment (thanks Piotr Dymacz)

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