ddr: marvell: only assert M_ODT[0] on write for a single CS

When using only a single DDR chip select only assert M_ODT[0] on write.
Do not assert it on read and do not assert M_ODT[1] at all. Also set
tODT_OFF_WR to 0x9 which contradicts the recommendation from the
functional spec but is what Marvell's binary training blob does and
seems to give better results when ODT is active during writes.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>