drivers/ddr/fsl: update workaround for erratum A-008511

Per the latest erratum document, update step 4 and step 8, only
DEBUG_29[21] is changed, all other bits should not be changed.

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