Add a new MicroPython board definition with slower flash clocks

In the production run of OpenCube, it was discovered that some of the
flash chips (3-4 bricks from 25) fail to run at the default 66 MHz.
It was separately tested that at 33 MHz, they work reliably.

To change the clocks, it is necessary to redefine the
PICO_FLASH_SPI_CLKDIV macro. To achieve that, this commits adds
a new MicroPython board based on the Pico reference board.
The PICO_FLASH_SPI_CLKDIV macro is changed via a trick copied from
the "nullbits Bit-C PRO" MicroPython board -- this board had a
custom board config header which includes the PICO_FLASH_SPI_CLKDIV
setting. This setting is now set to 4 to achieve the 33 MHz SPI freq.

However, the way this is done (adding a text-based patch on top of
a MicroPython submodule) is not sustainable in the long-term.
Eventually, this change should be migrated to the MicroPython upstream
(I don't think they would protest much against us adding basic OpenCube
support there). The board definition could also be improved by
specifying the pin names, changing the default pins, etc.