Add DMA support for USART

[hannobraun]

I've started implemented DMA support for USART. I'm submitting this as a draft, as it's not done yet.

Currently, only writing to USART via DMA is supported. Next week, I plan to implement reading from USART and add support for DMA interrupts.

@mvertescher If you have any thoughts so far, I'd love to hear them. I realize this is a complex API, but I believe the complexity is required to prevent incorrect use of the DMA peripheral through the HAL API.

[hannobraun]

I've changed my mind. I'm submitting this pull request as-is. The extensions I'm working on are a bit more involved than I thought, and there's really no point in overloading one pull request with all of them, when this is already useful as-is.

[mvertescher]

Spent a bit reviewing this PR and overall it seems consistent with stm32f7 family and other existing DMA implementations. As a result, I don't have any major concerns. Also, I didn't find the API too complex, however we may find ways to simplify it in the future after some use.

The extensions I'm working on are a bit more involved than I thought, and there's really no point in overloading one pull request with all of them, when this is already useful as-is.

Sounds like a good plan to me. This PR certainly provides value already.

I definitely like where this implementation is heading. Thanks for your work on this @hannobraun!