platform/chrome: wilco_ec: Add keyboard backlight LED support

The EC is in charge of controlling the keyboard backlight on
the Wilco platform. We expose a standard LED class device
named platform::kbd_backlight.

Since the EC will never change the backlight level of its own accord,
we don't need to implement a brightness_get() method.

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