[spi_host] Potential future enhancements

This are "nice to have" enhancements, and are not intended to gate ES.

- Since we have the ability to declare muxed and dedicated IOs in a more granular way now, we could add more than one CS signal per SPI-host and route them through the pinmux. this would allow the SPI-host to interface with multiple devices.
- We may want to consider adding either a status indication or error condition for cases where passthrough is enabled while SPI-host is active. Right now it is the task of SW to keep track of this at the system level. The SPI-host FSM is currently oblivious to passthrough enablement.

> estimate 8

Start by reviewing the SPI-host FSM and how passthrough enablement is handled. Clarify whether the intended scope is multiple chip-select routing, passthrough status or error reporting, or both; done should include an agreed design and corresponding implementation and verification.

Written by the indexing model from the issue text.