media: videodev2.h: add V4L2_FMT_FLAG_ENC_CAP_FRAME_INTERVAL flag

Add the V4L2_FMT_FLAG_ENC_CAP_FRAME_INTERVAL flag to signal that
the coded frame interval can be set separately from the raw frame
interval for stateful encoders.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>