Add section about DMA

[qwandor]

This could be an extension of the UART driver in the bare metal afternoon, as the PL011 appears to support DMA. This should cover:

• Using lifetimes for buffers to make a safe API (probably using 'static), why Drop is not enough (because core::mem::forget is safe).
• Pin
• Passing ownership back and forth with the type state pattern to prevent other operations clashing with the DMA transfer.
• Compiler and memory barriers (core::sync::atomic::compiler_fence and core::sync::atomic::fence).

https://docs.rust-embedded.org/embedonomicon/dma.html could be a useful source, and maybe some of the virtio-drivers crate.