SPMI controller docs

[mildsunrise]

I'm unlikely to work more on this, so here it is. explains

• how the FIFOs work generally (i.e. the stuff that is already implemented)
• format of reply words
• available interrupts
• interrupt controller functionality
• how to send each of the SPMI commands (not 100% of them... authenticate, transfer BO, device descriptor and master commands are still missing)
• some auxiliary functionality for debugging the FIFOs

unlike #220, this comes exclusively from hardware exploration