Provides an HFCLK timer driver

The existing (default) timer driver for nRF implements one using the RTC, which uses the LFCLK. An additional feature has been provided where the HFCLK can be used for the timer driver by using TIMER1. An STM example has also been ported so that either driver implementation can be exercised.

To use the HFCLK driver, simply use the `time-driver-timer1` feature in place of the `time-driver-rtc1` one when depending on embassy-nrf.

examples/nrf/src/bin/timer2.rs

@@ -0,0 +1,27 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+#[path = "../example_common.rs"]
+mod example_common;
+use defmt::assert;
+use embassy::executor::Spawner;
+use embassy::time::{Duration, Instant, Timer};
+use embassy_nrf::Peripherals;
+use example_common::*;
+
+#[embassy::main]
+async fn main(_spawner: Spawner, _p: Peripherals) {
+    info!("Hello World!");
+
+    let start = Instant::now();
+    Timer::after(Duration::from_millis(100)).await;
+    let end = Instant::now();
+    let ms = (end - start).as_millis();
+    info!("slept for {} ms", ms);
+    assert!(ms >= 99);
+    assert!(ms < 110);
+
+    info!("Test OK");
+    cortex_m::asm::bkpt();
+}