[media] s5p-mfc: Add support for EOS command and EOS event in video decoder

Add support for V4L2_DEC_CMD_STOP command which will instruct MFC device
to finish decoding and release all remaining frames kept for reference to
the user. After dequeueing last decoded frame the driver will generate an
V4L2_EVENT_EOS event.

Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Kyngmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>