board: freescale: ls1012a: Enable secure DDR on LS1012A platforms

PPA binary needs to be relocated on secure DDR, hence marking out
a portion of DDR as secure if CONFIG_SYS_MEM_RESERVE_SECURE flag
is set

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Abhimanyu Saini <abhimanyu.saini@nxp.com>
Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>