board: sl28: set CPO value

board: sl28: set CPO value

With a 8GiB memory board, it seems that the "very unlikely event" of a
DDR initialization with non-optimal values are not really that unlikely.
It happens in about every other reboot. As described in erratum
A-009942, preset the DEBUG_28 register with an optimal value. The value
iself depends on the memory configuration of the board, but the used
value seems to work well for all variants.

Signed-off-by: Michael Walle <michael@walle.cc>