Add an nRF RNG driver

Resolves #187

Like the stm32 driver, this has both a non-blocking and blocking API, and implements `rand_core::RngCore` for the blocking API.

examples/nrf/Cargo.toml

@@ -29,3 +29,4 @@ cortex-m-rt = "0.6.13"
 embedded-hal    = { version = "0.2.4" }
 panic-probe = { version = "0.2.0", features = ["print-defmt"] }
 futures = { version = "0.3.8", default-features = false, features = ["async-await"] }
+rand = { version = "0.8.4", default-features = false }

examples/nrf/src/bin/rng.rs

@@ -0,0 +1,30 @@
+#![no_std]
+#![no_main]
+#![feature(min_type_alias_impl_trait)]
+#![feature(impl_trait_in_bindings)]
+#![feature(type_alias_impl_trait)]
+#![allow(incomplete_features)]
+
+#[path = "../example_common.rs"]
+mod example_common;
+
+use defmt::panic;
+use embassy::executor::Spawner;
+use embassy_nrf::Peripherals;
+use embassy_nrf::rng::Rng;
+use embassy_nrf::interrupt;
+use embassy::traits::rng::Rng as _;
+use rand::Rng as _;
+
+#[embassy::main]
+async fn main(_spawner: Spawner, p: Peripherals) {
+    let mut rng = Rng::new(p.RNG, interrupt::take!(RNG));
+
+    // Async API
+    let mut bytes = [0; 4];
+    rng.fill_bytes(&mut bytes).await.unwrap(); // nRF RNG is infallible
+    defmt::info!("Some random bytes: {:?}", bytes);
+
+    // Sync API with `rand`
+    defmt::info!("A random number from 1 to 10: {:?}", rng.gen_range(1..=10));
+}