armv7: ls1021aqds: Set cpo_sample for erratum A-009942

Set cpo_sample as suggested by the driver
"WARN: pls set popts->cpo_sample = 0x58 in <board>/ddr.c to optimize
cpo".

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Shengzhou Liu <Shengzhou.Liu@nxp.com>