Support SPI DMA transfers

[mciantyre]

From @dflemstr:

I love the recent development and buzz around SPI support. In my application, I want to transfer a lot of data, and it would hence be useful to support DMA transfers for SPI!

I have another side project where I've been trying to eek out the maximum amount of performance from SPI in a device independent manner. I defined the API to look like this: https://github.com/dflemstr/embedded-platform/blob/master/src/spi.rs

Of course, that introduces async as well, while this crate/embedded_hal is very focused on blocking APIs. However the main take-away from that API I think is:

You initiate a transfer which takes ownership of the slice of memory you want to send
You can poll that transfer to completion (and do that in a tight loop if you want a "blocking" API)

Manually migrating this from mciantyre/teensy4-rs#51

[mciantyre]

Anyone mind if I take this on? I've been playing around with the DMA controller, and I have a working DMA-based UART driver in another project. My second pass should be better than my first 😛

[mciantyre]

@dflemstr, if you have the time, I'd love your thoughts on #49. I've demonstrated full-duplex SPI transfers using DMA in the teensy4-rs repo, and it'd be good to see if this could satisfy your use-case.

[dflemstr]

This is looking fantastic! Due to covid and other things I have had to postpone making progress, but I will make an attempt to port over the code for my use-case on top of your feature branch!