Merge branch 'feature/spi_slave' into 'master'

Split common SPI stuff out of master driver; add slave driver; add workaround for DMA issue.

This merge req mainly adds a slave device. In order to do this, the original master driver is refactored into common code shared by master and slave modes, and a slave driver is added.

The other things added are:
- Added a workaround for a 'feature' of the ESP32 silicon that can lock up the receive DMA channel in some situations. This can only be fixed by resetting *both* DMA channels. The workaround implemented makes sure that the reset only happens when both channels are idle
- Got rid of the automatic choice between register- and DMA-based transfers. The master (and slave) code will now always go for a DMA transfer if a DMA channel is given, and always go for register-based transfers if no DMA channel is given.
- Add in a bunch of fixes for outstanding Github issues.

See merge request !659