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>

board/kontron/sl28/ddr.c

@@ -54,6 +54,9 @@ static fsl_ddr_cfg_regs_t __maybe_unused ddr_cfg_regs = {
 
 	.ddr_cdr1		= 0x80040000,
 	.ddr_cdr2		= 0x0000bc01,
+
+	/* Erratum A-009942, set optimal CPO value */
+	.debug[28]		= 0x00700040,
 };
 
 int fsl_initdram(void)