stm32f4: add adc + example

Example tested on stm32f407vg Discovery Board.
minimal adc: no vref, dma, complex sequence

examples/stm32f4/src/bin/adc.rs

@@ -0,0 +1,26 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+#[path = "../example_common.rs"]
+mod example_common;
+
+use embassy::executor::Spawner;
+use embassy::time::{Delay, Duration, Timer};
+use embassy_stm32::adc::Adc;
+use embassy_stm32::Peripherals;
+use example_common::*;
+
+#[embassy::main]
+async fn main(_spawner: Spawner, p: Peripherals) {
+    info!("Hello World!");
+
+    let mut adc = Adc::new(p.ADC1, &mut Delay);
+    let mut pin = p.PC1;
+
+    loop {
+        let v = adc.read(&mut pin);
+        info!("--> {} - {} mV", v, adc.to_millivolts(v));
+        Timer::after(Duration::from_millis(100)).await;
+    }
+}