ramips: use rfkill for wps button on wlan only boards

All of the touched boards don't have an ethernet port. Allow to use the
wps button on these boards to enable the wireless radio after boot.

The force enabled wireless for the DCH-M225 is removed. It is reckless
to bring up an unencrypted wireless network by default these days.

Using the wps button to bring up the radio seam to me the better
approach.

Signed-off-by: Mathias Kresin <dev@kresin.me>