cam-service,doc: Use explicit padding in cam_message.h

Currently the structure cam_msg_a2s_start, cam_msg_a2s_stop and
cam_msg_a2s_event are using GCC attributes packed and aligned(8),
the latter is making these structure to have an implicit padding
of 4 bytes for cam_msg_a2s_start and cam_msg_a2s_stop and 6 bytes
for cam_msg_a2s_event.

In order to don't rely on the implicit padding, make them explicit
and update the documentation to list the reserved space of each
message.

Take also the occasion to update the "Stream Event Message" section
listing some missing field.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>