omx: Add split-field interlacing support to videoencoder/decoder

Submitted by Zeeshan Ali @zeenix

Link to original bug (#797061)

Description

These patches add split-field (alternate) interlacing support. Currently the implementation is specific to Zynq ultrascale+ target.

These patches are not yet ready to be merged as the caps lack the needed feature, which will be added soon. The point of submitting them already was to already get opinions/suggestions on the main changes.