media: venus: adding core part and helper functions

 * core.c has implemented the platform driver methods, file
operations and v4l2 registration.

 * helpers.c has implemented common helper functions for:
   - buffer management

   - vb2_ops and functions for format propagation,

   - functions for allocating and freeing buffers for
   internal usage. The buffer parameters describing internal
   buffers depends on current format, resolution and codec.

   - functions for calculation of current load of the
   hardware. Depending on the count of instances and
   resolutions it selects the best clock rate for the video
   core.

 * firmware loader

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>