* I'm trying to set the RP2350A's `TIMER0` to use `clk_sys` as its source.  This will change its readout register from incrementing at once per μs to once per `clk_sys` cycle (150MHz).

I'm setting `TIMER0`'s `source` and `lock`ing it, but my tests show that `TIMER0` is still incrementing at 1MHz.  I have verified that this code _is_ getting invoked (e.g. inserting a `panic!()` call will prevent the chip from booting).  Any ideas what I might be missing?

```rust
#[pre_init]
unsafe fn pre_init() {
    // SAFETY: `#[pre_init] fn` runs before stage 1 bootloader, still in privileged thread mode.
    //      Any access of memory, including any static variables, will result in undefined behavior.
    //      (see https://docs.rs/cortex-m-rt/latest/cortex_m_rt/attr.pre_init.html for more details)
    let timer_0 = rp_pac::TIMER0;

    // Switch `SOURCE` to `CLK_SYS` (instead of 1 µs tick)
    timer_0.source().write(|source| source.clk_sys());

    // Lock configuration--`TIMER0` now read-only (until reset)
    timer_0.locked().write(|lock| lock.set_locked(true));
}
```

From https://datasheets.raspberrypi.com/rp2350/rp2350-datasheet.pdf, §12.8.5, p.1186, offset 0x38: `Source`.