arm: mxs: olinuxino: Fine-tune DRAM configuration

Add fine-tuning for the DRAM configuration according to the DRAM chip
datasheet. THis configuration applies to both Hynix HY5DU12622DTP and
Samsung K5H511538J-D43 .

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>