mxs: spl_mem_init: Align DDR2 init with FSL bootlets source

Currently the following kernel hang happens when loading a 2.6.35 kernel from
Freeescale on a mx28evk board:

RPC: Registered tcp transport module.
RPC: Registered tcp NFSv4.1 backchannel transport module.
Bus freq driver module loaded
IMX usb wakeup probe
usb h1 wakeup device is registered
mxs_cpu_init: cpufreq init finished
...

Loading the same kernel using the bootlets from the imx-bootlets-src-10.12.01
package, the hang does not occur.

Comparing the DDR2 initialization from the bootlets code against the U-boot
one, we can notice some mismatches, and after applying the same initialization
into U-boot the 2.6.35 kernel can boot normally.

Also tested with 'mtest' command, which runs succesfully.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Tested-by: Marek Vasut <marex@denx.de>