MLK-16069 imx6slevk: Workaround to limit the u-boot in low 512MB memory

MLK-16069 imx6slevk: Workaround to limit the u-boot in low 512MB memory

On i.MX6SLEVK board, the LPDDR2 chip(CS1) is not reset before accessing.
And due to MMDC limitation, the script we get from IC team is only doing
CS0 reset but skipping CS1 reset, the reason is that doing CS1 reset might
cause CS0 can NOT be accessed any longer.

Because of this HW issue, we found the high 512MB memory needs more time to
be stable. Since the u-boot relocates itself to highest address after booting,
so this will cause issue.
To work around it, we just limit the u-boot running at low 512MB memory.

Signed-off-by: Ye Li <ye.li@nxp.com>