ramips: switch to gpio-button-hotplug

This, in theory at least, saves us a bit of RAM
by not having to load in the input subsystem.

Tested on AR725W and WRT160Nv2, but should work
everywhere.

Signed-off-by: Claudio Leite <leitec@staticky.com>
Merge branch 'master' of github.com:leitec/openwrt-leitec