Support video decoding using the VPU (using gstreamer)

Don't confuse this with https://source.puri.sm/Librem5/linux-next/issues/68 which only makes sure it's disabled until we can close this very issue (have support).

• Test and enhance what is being worked on upstream: https://lore.kernel.org/linux-media/?q=hantro
• Testing: most probably using gstreamer's gst-launch-1.0 (zero-copy, performance debugging, ...)
• Also important: Document what exact codecs and containers are supported in what exact levels. i.e. what video files users can throw at it.
• nice-to-have: Have good support for a free codeoc like VP9