Design SPI protocol between MCU1 and MCU2

[kbeckmann]

In the v2 concepts we need pico1 (connected to the n64 bus) to load/send:

• sram data
• wifi data
• sdcard data
• debug data
• etc...

via MCU2.

Start prototyping on a communication solution for this.

[kbeckmann]

Look at src/rp2_common/cyw43_driver/cyw43_bus_pio_spi.c + cyw43_bus_pio_spi.pio for SPIO with a shared MISO/MOSI pin.