Add draft HDCP version registry

[chrisn]

Following from #515, this adds a draft registry for HDCP versions.

Still to do:

• [x] Add references for HDCP 2.0 and upwards
• [x] Add spec links
• [ ] Update EME spec to reference the registry

[chrisn]

Thank you all for reviewing. I suggest making a new PR to update the EME spec to reference the registry, so will go ahead and merge this.