MLK-20113 spi: ecspi: add pio spi slave mode for imx6ul

MLK-20113 spi: ecspi: add pio spi slave mode for imx6ul

Add pio spi slave mode support for imx6ul which is based on the patch
cherry-pick from community(71abd29057cb17b6b9532421821dc443427399ed).

Because the original imx6ul is bound to mx51 in the dts file.
However, in the slave mode, the patch in the community only adds two
functions mx53_ecspi_rx_slave/mx53_ecspi_tx_slave. Therefore, configure
mx51/6ul to use these two functions for data transmission and reception
in slave mode.

Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Signed-off-by: Arulpandiyan Vadivel <arulpandiyan_vadivel@mentor.com>