imx8qm_dmsse20a1: Improve SPL memory configuration

Currently, building imx8qm_dmsse20a1_defconfig leads to the following
warning:

aarch64-linux-ld.bfd: invalid origin for memory region .sdram

Fix it by using the same SPL memory layout as done in the imx8mq_evk.

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Tested-by: Oliver Graute <oliver.graute@kococonnector.com>