Provides an HFCLK timer driver

[huntc]

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.

[huntc]

• Could you split into several files? time_driver/{mod.rs, rtc.rs, timer.rs}

Resolved in commit d99c74807a5b09fad79702aae1ba686bf8a1d8d0