ARM: tegra: acer-a500: Add Embedded Controller

This patch adds device-tree node for the Embedded Controller which is
found on the Picasso board. The Embedded Controller itself is ENE KB930,
it provides functions like battery-gauge/LED/GPIO/etc and it uses firmware
that is specifically customized for the Acer A500 device.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>