omxvideoenc: Add long-term-ref support to ZYNQ_USCALE_PLUS encoder

omxvideoenc: Add long-term-ref support to ZYNQ_USCALE_PLUS encoder

Custom API that upstream elements can use to notify encoders about
marking longterm ref. pictures or using longterm ref. pictures in
encoding process.

This patch adds below properties:

long-term-ref: Enable/Disable dynamically marking long-term
reference pictures in encoding process

long-term-freq: Periodicity of long-term reference picture
marking in encoding process.

If a picture is marked as long-term reference picture then it remains
in the DPB list for ever unless it overrides with new long-term pitcure with
same index. Encoder can use this long-term picture as refence for
encoding.

This feature is mostly useful to avoid visual artifacts propagation in streaming use cases
when packet loss happens. Instead of requesting for IDR, client can request for use long-term
reference picture for encoding.