rp: add single-channel dma from adc

with uniform treatment of adc inputs it's easy enough to add a new
sampling method. dma sampling only supports one channel at the moment,
though round-robin sampling would be a simple extension (probably a new
trait that's implemented for Channel and &[Channel]). continuous dma as
proposed in #1608 also isn't done here, we'd expect that to be a
compound dma::Channel that internally splits a buffer in half and
dispatches callbacks or something like that.

examples/rp/src/bin/adc.rs

@@ -25,7 +25,7 @@ async fn main(_spawner: Spawner) {
     let mut p26 = Channel::new_pin(p.PIN_26, Pull::None);
     let mut p27 = Channel::new_pin(p.PIN_27, Pull::None);
     let mut p28 = Channel::new_pin(p.PIN_28, Pull::None);
-    let mut ts = Channel::new_sensor(p.ADC_TEMP_SENSOR);
+    let mut ts = Channel::new_temp_sensor(p.ADC_TEMP_SENSOR);
 
     loop {
         let level = adc.read(&mut p26).await.unwrap();