[atheros] [mips] Add the GPIO driver (back) to the TL-WDR3600/TL-WDR4300 kernel.

So it turns out that sometime in the past I removed the GPIO bits here
and was going to move it into a module in order to save a little space.
However, it turns out that was a mistake on this particular AP - it
uses a pair of GPIO lines to control the two receive LNAs on the 2GHz
radio and without them enabled the radio is a LOT DEAF.

With this re-introduced (and some replacement userland tools to save
space, *cough* cpio/libarchive) I can actually use these chipsets
again as a 2G station. Without the LNA the AP was seeing a per-radio
RSSI upstairs here of around 3-5dB, with the LNA on it's around 15dB,
more than enough to actually use wifi upstairs and also in line with
the other Atheros / Intel devices I have up here.

Big oopsie to Adrian. Big, big oopsie.