board/ls1021aqds: Add DDR4 support

LS1021AQDS has a variant with DDR4 slot. This patch adds a new defconfig
for this variant to enable DDR4 support. RAW timing parameters are not
added for DDR4. The board timing parameters are only tuned for single-
rank 1600 and 1800MT/s with Micron DIMM 9ASF51272AZ-2G1A1 due to DIMM
availability.

Signed-off-by: York Sun <yorksun@freescale.com>
CC: Alison Wang <alison.wang@freescale.com>