Documentation: how to use the ECP5 PLL

[danderson]

I wrote some documentation on how to use the Lattice ECP5's PLL hardware block: https://blog.dave.tf/post/ecp5-pll/. I believe it's a correct and (almost) complete description of all the primitive's configuration parameters and I/O signals.

I verified the behaviors I documented by comparing to the configurations generated by Lattice Diamond, as well as empirically by loading test designs onto my ECP5 and poking it with an oscilloscope.

If you're hacking on an ECP5 design that needs additional clocks, hopefully you'll find this useful!