travis-ci: add html deployment to github pages
travis-ci: add html deployment to github pages

The built html output will be pushed back to gh-pages branch which in
turn is used for http://devicetree-org.github.io/devicetree-specification/
website.

There's some rendering problems likely due to github pages setup that
need to be solved.

Signed-off-by: Rob Herring <fbb37eacae100e13076d5c4b46f06cfa4d6fa9f8@kernel.org>

devicetree-org/devicetree-specification