km/ls102xa: add support for u-boot POST memory test

From production view this is standard test executed during production on
all linux based foxmc cards.
On CENT2 HW defined memory region is zero means that some DDR accesses are
done by memory_post_dataline and memory_post_addrline but pattern tests
are skipped that's why mem_regions is fast there.

On ls102x for the complete DDR region of 1GiB memory_regions_post_test
takes approx. 4min and this is too much for production, so this patch
defines only 1MiB region as compromise.

Signed-off-by: Aleksandar Gerasimovski <aleksandar.gerasimovski@hitachi-powergrids.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>