mfd / platform: cros_ec: Miscellaneous character device to talk with the EC

That's a driver to talk with the ChromeOS Embedded Controller via a
miscellaneous character device, it creates an entry in /dev for every
instance and implements basic file operations for communicating with the
Embedded Controller with an userspace application. The API is moved to
the uapi folder, which is supposed to contain the user space API of the
kernel.

Note that this will replace current character device interface
implemented in the cros-ec-dev driver in the MFD subsystem. The idea is
to move all the functionality that extends the bounds of what MFD was
designed to platform/chrome subsystem.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Tested-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>