Implement and evaluate PSRAM QSPI controller

[kbeckmann]

First, just implement something that can write and read the PSRAM.

Write test code that fills whole PSRAM with pseudo-random numbers, then verify.

Measure performance, latency.

Investigate how to tie it in to the n64 bus PIO.

[kbeckmann]

Latency test:

Set up a test harness that, when a trigger gpio goes high, loads 2 bytes from the PSRAM, outputs the result on gpio0->gpio15. Measure latency with a logic analyzer.

[kbeckmann]

Not relevant anymore