DIB: Add parallel power option between slot2 and slot3 on DIB backplane

[fietser28]

I would like to have the possibility to have parallel operation of modules in slot 2 and slot 3.
This creates more flexibility to combine (future) modules like electronic load.

Low priority for now.

[prasimix]

This will also require changing the I/O expander because all 8 ports are used on the existing one.