[spi_device] interrupts for unhandled flash commands

When working on the QEMU SPI flash emulation I've noticed that our tests are receiving a lot of flash commands which we haven't configured slots for. Most commonly this will be the `WREN` and `EN4B`/`EX4B` commands.

QEMU has a helpful debug trace to warn about unhandled commands, but I was surprised the hardware doesn't have anything similar. I was expecting to find an interrupt or some kind of status register to tell Ibex when an unhandled flash command has arrived.

This would mostly be useful for logging warnings and finding bugs.

The issue mentions QEMU SPI flash emulation, unhandled WREN and EN4B/EX4B commands, and Ibex. Start by tracing the existing SPI device command handling and checking how interrupts or status registers are represented. Done means the desired handling for unconfigured flash commands is designed and documented well enough to implement.

Written by the indexing model from the issue text.