Add embedded_hal_async support for embassy-rp

This commit adds support for embedded-hal-async to the Embassy
Raspberry PI crate.

examples/rp/src/bin/gpio_async.rs

@@ -0,0 +1,38 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use defmt::*;
+use embassy::executor::Spawner;
+use embassy::time::{Duration, Timer};
+use embassy_rp::{gpio, Peripherals};
+use gpio::{Input, Level, Output, Pull};
+use {defmt_rtt as _, panic_probe as _};
+
+/// This example shows how async gpio can be used with a RP2040.
+///
+/// It requires an external signal to be manually triggered on PIN 16. For
+/// example, this could be accomplished using an external power source with a
+/// button so that it is possible to toggle the signal from low to high.
+///
+/// This example will begin with turning on the LED on the board and wait for a
+/// high signal on PIN 16. Once the high event/signal occurs the program will
+/// continue and turn off the LED, and then wait for 2 seconds before completing
+/// the loop and starting over again.
+#[embassy::main]
+async fn main(_spawner: Spawner, p: Peripherals) {
+    let mut led = Output::new(p.PIN_25, Level::Low);
+    let mut async_input = Input::new(p.PIN_16, Pull::None);
+
+    loop {
+        info!("wait_for_high. Turn on LED");
+        led.set_high();
+
+        async_input.wait_for_high().await;
+
+        info!("done wait_for_high. Turn off LED");
+        led.set_low();
+
+        Timer::after(Duration::from_secs(2)).await;
+    }
+}