mfd: cros-ec: Increase maximum mkbp event size

Having a 16 byte mkbp event size makes it possible to send CEC
messages from the EC to the AP directly inside the mkbp event
instead of first doing a notification and then a read.

Signed-off-by: Stefan Adolfsson <sadolfsson@chromium.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>