platform/chrome: wilco_ec: Add debugfs test_event file

This change introduces a new debugfs file 'test_event' that when written
to causes the EC to generate a test event.
This adds a second sub cmd for the test event, and pulls out send_ec_cmd
to be a common helper between h1_gpio_get and test_event_set.

Signed-off-by: Daniel Campello <campello@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>
Reviewed-by: Nick Crews <ncrews@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>