mfd: cros_ec: Switch to use the new cros-ec-chardev driver

With the purpose of remove the things that far extends the bounds of
what a MFD was designed to do, instantiate the new platform misc
cros-ec-chardev driver and get rid of all the unneeded code. After this
patch the misc chardev driver is a sub-device of the MFD, and all the
new file operations should be implemented there.

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>