configs: ls1021a: Reserve low memory for CMA

The default reserved memory for CMA is high memory. If LPAE is enabled,
highmem pages are non-remapped and can not be used with
dma_alloc_coherent. This patch will reserve low memory for CMA and fix
the issue on LS1021A.

Signed-off-by: Peng Ma <peng.ma@nxp.com>
Signed-off-by: Shengzhou Liu <Shengzhou.Liu@nxp.com>
Signed-off-by: Alison Wang <alison.wang@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>