Basic Hotplug FPGA Support

[rmustacc]

This covers adding support for the hotplug FPGA which needs to:

• Speak some combination of PCA9506/9535 as an i2c target from SP5 (likely multiple PCA9506s)
• Provide a SPI interface to the SP for its own interrogation and out of band shenanigans (more in a moment)
• Implement all of the basic discrete logic that we require to power hotplug and generate alerts back to MPIO

Longer term we will want to find different ways to use this for testing and related (simulating hotplug without actually doing hotplug).

[rmustacc]

Note, on the main FPGA this also covers the PERST timer logic that is no longer implemented with discrete parts. However, it still is on the sharkfins.