ARM: dts: imx6: Use correct SDMA script for SPI cores

According to the reference manual the shp_2_mcu / mcu_2_shp
scripts must be used for devices connected through the SPBA.

This fixes an issue we saw with DMA transfers from SPI NOR Flashes.
Sometimes the SPI controller RX FIFO was not empty after a DMA
transfer and the driver got stuck in the next PIO transfer when
it read one word more than expected.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>