mfd: cros_ec: Use mfd_add_hotplug_devices() helper

Use mfd_add_hotplug_devices() helper to register the subdevices. The
helper allows us to reduce the boiler plate and also registers the
subdevices in the same way as used in other functions used in this
files.

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