Add support for DHT11 driver

The DHT11 is a temperature and humidity sensor supported by linux
since 3.14.

This patch is tested on recent mxs with kernel 3.14.18.

Signed-off-by: Harald Geyer <harald@ccbib.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43103 3c298f89-4303-0410-b956-a3cf2f4a3e73