platform/chrome: cros_ec: Add SW_FRONT_PROXIMITY MKBP define

platform/chrome: cros_ec: Add SW_FRONT_PROXIMITY MKBP define

Some cros ECs support a front proximity MKBP event via
'EC_MKBP_FRONT_PROXIMITY'. Add this define so it can be used in a
future patch.

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Benson Leung <bleung@chromium.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Gwendal Grignou <gwendal@chromium.org>
Acked-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20210211024601.1963379-2-swboyd@chromium.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>