MLK-19789 mach-imx: imx8: Make the eMMC container offset SOC dependant

MLK-19789 mach-imx: imx8: Make the eMMC container offset SOC dependant

This is a hack for imx8qm-mek, since the offset of the flash.bin image
on eMMC differs when compared to imx8qxp-mek. Basically, the default value
is 32K, but for 8qm-mek it's 0. This can go away once the qm and qxp get
aligned (again) from this point of view.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit e9f87deae7e8cb3e71012f85c488e0a0d108762a)