imx8mp-venice: fix DRAM bus configuration

The DRAM configuration for the 1GB and 4GB imx8mp venice boards had a
bus mapping issue (channel A and B swapped) which creates an invalid
deskewing configuration during training causing the DRAM to not be able
to run at its full bus speed.

Update the various config structures to resolve this.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>