I've found an 'odd' quirk.  If I compile the below code and get the binary at speeds > 420Mhz, the file-size is pretty "invalid".  What I mean by this is that a normal binary is ~138kB, but an invalid one compiles down to 11kB (at the higher speed).  Obviously the latter case isn't going to run.

The M7 chip is rated to 480MHz at its boosted operation though.

```rust
    #[cfg(feature = "giga_r1_wifi")]
    let mut pwrcfg = pwr.ldo().vos0(&dp.SYSCFG).freeze();

    #[allow(unused_variables)]
    let backup = pwrcfg.backup().unwrap();

    #[cfg(feature = "giga_r1_wifi")]
    // Constrain and Freeze clocks
    let ccdr = {
        let mut ccdr = rcc
            .use_hse(16.MHz())
            // FIXME speeds > 420MHz seem to fail!
            .sys_ck(420.MHz())
```