Ignition Controller: Repurpose PCIE_HOST_TO_FPGA_PWREN to PCIE_FPGA_TO_HOST_ALERT_L

[Aaron-Hartwig]

This means renaming, swapping the input to an output, and adding a weak internal pull-up on the D5 pin.

[Aaron-Hartwig]

Note that this change needs to be released with care. On current hardware, this pin is meant to be an input and is being driven by a buffer, meaning if we were to just swap the FPGA pin here it would cause the pin and buffer to fight, ultimately ending in sadness. This can only happen for Build D hardware (does not exist yet) or previous builds which have been modified. When I write the MCN I will reference it here.

[Aaron-Hartwig]

See updates and notes to https://github.com/oxidecomputer/hardware-sidecar/issues/765, but the action here going forward is just to remove the PCIE_HOST_TO_FPGA_PWREN pin altogether.