Fix clock setup for MSI and PLL to allow RNG opereation

Add RNG example using PLL as clock source.

examples/stm32l4/src/bin/rng.rs

@@ -0,0 +1,37 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+#[path = "../example_common.rs"]
+mod example_common;
+use embassy::executor::Spawner;
+use embassy::time::{Duration, Timer};
+use embassy::traits::rng::Random;
+use embassy_stm32::rcc::{ClockSrc, PLLClkDiv, PLLMul, PLLSource, PLLSrcDiv};
+use embassy_stm32::rng::Rng;
+use embassy_stm32::{Config, Peripherals};
+use example_common::*;
+
+fn config() -> Config {
+    let mut config = Config::default();
+    config.rcc = config.rcc.clock_src(ClockSrc::PLL(
+        PLLSource::HSI16,
+        PLLClkDiv::Div2,
+        PLLSrcDiv::Div1,
+        PLLMul::Mul8,
+        Some(PLLClkDiv::Div2),
+    ));
+    config
+}
+
+#[embassy::main(config = "config()")]
+async fn main(_spawner: Spawner, p: Peripherals) {
+    info!("Hello World!");
+
+    let mut rng = Random::new(Rng::new(p.RNG));
+
+    loop {
+        info!("random {}", unwrap!(rng.next_u8(16).await));
+        Timer::after(Duration::from_secs(1)).await;
+    }
+}