P1 bringup: Rationalise powering

The power scheme for the board could be rationalised/cleaned up.

To make the schematic clearer, the "analogue" regulators (bottom right of sheet) should be moved to their own sheet. Each IRPS could also have its own sheet.

There are also some unused channels on the IRPS which could possibly be used to instead of separate switchers e.g 1V8, 3V3.

Also note #126, we require an external 5V for the IRPS.