Add support for 3-wire SPI

[avsa242]

The chip can operate over 3-wire SPI as a compromise between lesser I/O pins used I2C and faster 4-wire SPI.

[avsa242]

Update: This can be done, but isn't as trivial as it was originally thought to be. The 4MHz SPI engine can't really support 3-wire SPI as-is. The input/output pinmasks are set at engine start, and not touched at the read/write transaction level, as in some other engines, so the same pin can't be for example an output and then an input. For this support, the SPI engine will need to be switched to the 1MHz engine.