ARM: imx: Do not hard-code MX8M MMU table DRAM entry offset

Instead of hard-coding the offset of DRAM entries in MMU table all over
the code, auto-detect the offset. This removes error-prone code which
would break e.g. in case the MMU table is modified.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>