MAINTAINERS: use the correct location for dt-bindings includes for mux

Just drop the "linux" part of the path, it was never correct.

Reported-by: Joe Perches <joe@perches.com>
Fixes: 256ac037

 ("dt-bindings: document devicetree bindings for mux-controllers and gpio-mux")
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>