peci: Convert eSPI systems to use PECI-over-eSPI

[crawfxrd]

Is this possible, or will the H_PECI pin cause issues as it's connected to the CPU?
Are there any changes needed for coreboot/FSP?

The changes would be:

• Change H_PECI to be an unused pin: input and pulled down
• Select PECI_OVER_ESPI for boards using eSPI, or rather, make it the default

Requires resolving #369.