There is such a description in the data sheet：
188.8.131.52.2 Master Mode with Wait States
Wait states can be inserted between SPI bursts. This provides a way for software to slow down the SPI burst to meet the timing requirements of a slower SPI device.
The following figure shows wait states inserted between SPI bursts.
In this case, the number of wait states is controlled by ECSPI_PERIODREG[SAMPLE PERIOD] and the wait states' clock source is selected by ECSPI_PERIODREG[CSRC]
184.108.40.206.3 Master Mode with SS_CTL[3:0] Control
The SPI SS Control (SS_CTL[3:0]) controls whether the current operation is single burst or multiple bursts.
When the SPI SS Wave Form Select (SS_CTL[3:0]) is set, the current operation is multiple bursts transfer. When the SPI SS Wave Form Select (SS_CTL[3:0]) bit is cleared, the current operation is single burst transfer. A SPI burst can contains multiple words as defined in the BURST LENGTH field of the ECSPI_CONREG register.
In Figure 20-8, two 8-bit bursts in the TXFIFO have been combined and transmitted in one SPI burst. The maximum length of a single SPI burst is defined in the BURST LENGTH field of the ECSPI_CONREG control register. (Figure 20-8 corresponds to a BURST LENGTH of 8.) This provides a way for transferring a longer SPI burst by writing data into TXFIFO while the ECSPI is transmitting.
But I did not find the ECSPI_PERIODREG register definition in spi-imx.c.