Merge #559

559: USART DMA example for the STM32F7 r=Dirbaio a=olofwalker

This small PR adds the USART DMA write an example for the STM32F7. The example has been tested on a Nucleo-f767zi board.

Output from `DEFMT_LOG=info cargo run --bin usart_dma`
```
Running `probe-run --chip STM32F767ZITx target/thumbv7em-none-eabihf/debug/usart_dma`
(HOST) INFO  flashing program (108 pages / 108.00 KiB)
(HOST) INFO  success!
────────────────────────────────────────────────────────────────────────────────
0 INFO  Hello World!
└─ usart_dma::__cortex_m_rt_main @ src/bin/usart_dma.rs:39
1 INFO  wrote DMA
└─ usart_dma::main_task::task::{generator#0} @ src/bin/usart_dma.rs:31
```

Co-authored-by: Robert Walker <rgit@walker.st>

examples/stm32f7/src/bin/usart_dma.rs

@@ -0,0 +1,30 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+#[path = "../example_common.rs"]
+mod example_common;
+use core::fmt::Write;
+use embassy::executor::Spawner;
+use embassy_stm32::dma::NoDma;
+use embassy_stm32::usart::{Config, Uart};
+use embassy_stm32::Peripherals;
+use embassy_traits::uart::Write as _Write;
+use example_common::*;
+
+use heapless::String;
+
+#[embassy::main]
+async fn main(_spawner: Spawner, p: Peripherals) {
+    let config = Config::default();
+    let mut usart = Uart::new(p.UART7, p.PA8, p.PA15, p.DMA1_CH1, NoDma, config);
+
+    for n in 0u32.. {
+        let mut s: String<128> = String::new();
+        core::write!(&mut s, "Hello DMA World {}!\r\n", n).unwrap();
+
+        unwrap!(usart.write(s.as_bytes()).await);
+
+        info!("wrote DMA");
+    }
+}