bindings: mfd: cros ec: Document vbc EC property

Some EC implementations include a small nvram space used to store
verified boot context data. This boolean property lets us indicate
whether this space is available or not on a specific EC implementation.

Signed-off-by: Emilio López <emilio.lopez@collabora.co.uk>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>