a10-dram-timings-calculator.rb: Derive from JEDEC Speed Bin DDR3-1333H

Use JEDEC Speed Bin DDR3-1333H as base timings. And then apply
individual tweaks specific to each DRAM chip (usually slightly
better timings than guaranteed by the standard bin).
a10-dram-timings-calculator.rb: Honour extra RK30XX requirements

The RK30XX manual lists some extra requirements for the timing
parameters. It's better to do what it says.