prepare for cPCIS mechanics

[jordens]

Would be great to have a future cPCIS port be easy without too much layout changes.

• There seems to be a natural superset of the cPCIS mechanics and our old 3U mechanics.
• I'd take the cPCIS mechanical template (handle, ejector, ESD strips, keepout, backplane connector area etc) and adhere to that.
• Then, when doing non-cPCI panels and mechanics, just don't cut the PCB.
• Put the IDC connectors close (!) together in bottom right corner and right edge. Make the layout switch from IDC to backplane connector easy.

c.f. https://github.com/sinara-hw/meta/issues/56#issuecomment-671469588

[gkasprow]

@jordens my concept was to follow the Kasli layout and use Kasli/Kasli SoC to CPCIs adapter. It will give us transition time to support both ribbon and CPCIS users. Some day we will switch to CPCIS with all modules. The KasliSoc is already route, Anyway, there will be a second revision, so I'd produce it as it is (with only minor changes)