add ability to statically assign DMA channels

[carlk3]

Please add some information about this new feature to the README. Specifically, in the An instance of spi_t describes the configuration of one RP2040 SPI controller. section.

Thanks

[carlk3]

Also, I think a better name the member name static_dma_channels would be use_static_dma_channels or maybe use_static_DMA_channels for consistency with use_exclusive_DMA_IRQ_handler .

[carlk3]

Also, I wanted to make sure you this wouldn't break other peoples existing usage code. My understanding is that it shouldn't, as long as their spi_t structs are intialized to zero. Which should be the case if they are globaly or staticly defined. What do you think?

That should be fine. I already have a remark about that in the README:

• Generally, anything not specified will default to 0 or false. (This is the user's responsibility if using Dynamic Configuration, but in a Static Configuration [see Static vs. Dynamic Configuration], the C runtime initializes static memory to 0.)