mfd: cros_ec: differentiate SCP from EC by feature bit

mfd: cros_ec: differentiate SCP from EC by feature bit

System Companion Processor (SCP) is Cortex M4 co-processor on some
MediaTek platform that can run EC-style firmware. Since a SCP and EC
would both exist on a system, and use the cros_ec_dev driver, we need to
differentiate between them for the userspace, or they would both be
registered at /dev/cros_ec, causing a conflict.

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Acked-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>