Is there a Linux driver that supports using the FlexSPI interface and a Shift Register device, 74xx595?
The 74xx595 GPIO driver works over any “standard SPI” ( ecspi in case of Verdin iMXMmini). So you can add it to desired interface similar as it done here.
However FlexSPI (aka QuadSPI) is designed for interfacing NAND and NOR flashes. Though it could be possible to interface 74xx595 through QuadSPI. currently it’s not supported.