media: allegro: make frame rate configurable

The allegro dvt codec adjust the encoding speed according to a
configured frame rate. Furthermore, the frame rate is written into the
coded stream.

Ensure that the coded video data has the correct frame rate by
implementing s_parm for setting the frame rate from user space.

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