mfd / platform: cros_ec: Rename config to a better name

The cros-ec-dev is a multifunction device that now doesn't implement any
chardev communication interface. MFD_CROS_EC_CHARDEV doesn't look
a good name to describe that device and can cause confusion. Hence
rename it to CROS_EC_DEV.

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>