dt-bindings: mfd: Add ENE KB3930 Embedded Controller binding

Add binding document for the ENE KB3930 Embedded Controller.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>