P1 bringup: Add switch/monitor to expansion board 12V

12V to the expansion board comes straight from the main PSU. The other rails (P3V3_EXP, P1V8_EXP) are derived from 12V by a separate switcher, which is enabled by the Zynq.

To avoid an issue on an expansion board causing a fault on the main 12V rail to the whole switch, we could add a high side switch/eFuse to the expansion board 12V rail. It could also be nice to have current monitoring for all the expansion board rails.