Add a tufty2350 target and board file for the Pimoroni Tufty 2350,
an RP2350B board with a parallel ST7789 LCD and a CYW43439 wireless
chip. The target inherits rp2350b and sets a 16 MB flash size, like
the existing pga2350 and pico-plus2 targets.
Pin names come from Pimoroni's published pins.csv for the board:
https://github.com/pimoroni/tufty2350/blob/main/board/pins.csv
Dave Cheney