mfd: cros: add "base attached" MKBP switch definition

This adds a "base attached" switch definition to the MKBP protocol that
is used by Whiskers driver to properly determine device state (clamshell
vs tablet mode).

Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>