Generate MRx values according to DRAM chip specs

[jeanthom]

Currently we use hardcoded values of the MR registers that only work on the ECPIX-5. We should either:

• set MR registers at compile time (leaner code, smaller ROM footprint)
• allow loading MR registers from a struct of some sort (needs a bit more code but could allow for a single firmware image to work on multiple boards with different RAM chips)

[jeanthom]

This has been partially fixed in https://github.com/jeanthom/gram/commit/bae4da5edf0b881db8f8731b6a4a5f7c7a1d9529, we now have to generate those MRx values.