i have actually a MCU connected to imx6q, in the classic MISO->MISO, MOSI->MOSI. Actually Linux is master and MCU is salve. All works fine. I would try, and as far as i know, with same wiring it should be possible, an inversion of role. Linux slave and MCU master. About MCU side thing is simple. About linux seems slave mode is not supported. Is this possible ? I found also a possible patch:
Any help to have imx6q as SPI configured as slave is appreciated.