QSPI support #80
Description
We should consider changing the Pico <--> AD9959 comms link to use the QSPI interface. This should reduce programming time for all commands by a factor of 4.
Primary hurdle is that the RP2040/RP2350 dedicated SPI hardware does not support the protocol so a PIO needs to be dedicated to the effort. I can't quite remember now, but I think it has to be a whole core do to instruction limits (ie I don't think we can use the extra state machines we have right now). The RP2350 has 3 PIO cores, so that solves the problem trivially, though confirming we can be more efficient in out use of PIO for the RP2040 would be warranted.
Note: pretty sure the QSPI implementation on the Pico exists somewhere, so we'd just need to copy paste. Testing and validation will be the hard part.