imx8: allow overriding memory layout

Introduce weak function board_mem_get_layout() which allows overriding
the memory layout from board code in runtime, useful for handling
different SKU versions.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>