add support for de10_nano with mistral toolchain

[infphyny]

I set 60 cores for now. The highest score I was able to achieve with all cores detected by Corey is 69, but since place and route is different between each build, sometimes a fraction of the 69 cores do not work. Will put in draft mode until I find a score that is reliable.

[infphyny]

74 is the maximum I was able to achieve. With the soc running at 12.5 MHz instead of 50 MHz, Corey count all the cores. Very good result. I saw Ravenslofty is working on better alm packing for nextpnr-mistral.

[infphyny]

The maximum number of cores is 84 with latest nextpnr-mistral.