mfd: cros_ec_dev: Add GPIO device if feature present on EC

mfd: cros_ec_dev: Add GPIO device if feature present on EC

The ChromeOS embedded controller (EC) supports setting the state of
GPIOs when the system is unlocked, and getting the state of GPIOs in all
cases. Check for the feature support by checking for the GPIO feature
and then populate a sub-device for the gpio hardware on the EC.

Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20240219202325.4095816-3-swboyd@chromium.org
Signed-off-by: Lee Jones <lee@kernel.org>