media: cros-ec-cec: Support receiving messages from multiple ports

Currently, received messages are sent from the EC in the cec_message
MKBP event. Since the size of ec_response_get_next_data_v1 is 16 bytes,
which is also the maximum size of a CEC message, there is no space to
add a port parameter. Increasing the size of
ec_response_get_next_data_v1 is an option, but this would increase
EC-kernel traffic for all MKBP event types.

Instead, use an event to notify that data is ready, and add a new read
command to read the data.

For backwards compatibility with old EC firmware, continue to handle
cec_message events as well.

Signed-off-by: Reka Norman <rekanorman@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>