Please add SPI Slave mode

I would like to interface with a RPI, so that RPI sends image data via SPI to Maix board, Maix board does face recognition and sends back result to RPI.

As RPI only has pins for SPI master mode, I need to use Maix board as SPI slave. But this is not yet implemented in MaixPy (according to the documentation). So, please add this.

Thanks!