More flexible clocking #76
Description
It would be nice to more fully support the faster clock speeds the Pico/Pico2 can provide (right now 200/150MHz respectively). It is tricky since using the Pico system clock to time the AD9959 pretty much fixes things to 125MHz anyway. But if the AD9959 is externally timed we could leverage the higher clock speeds to improve SPI/PIO times a small amount to generally improve timing resolution. For example, 4 channel single stepping updates would go from 10us to 6.25us/8.333us, respectively. Longer term, it seems likely the Pico2 will get the same treatment as the Pico and double its supported clock as well to 300MHz, giving 4.166us updates.