Add conversions from us/ns to Hertz

[mattico]

No description provided.

[hargoniX]

LGTM, although every HAL adding their own src/timer.rs which provides a U32Ext trait seems kinda dumb...we should definitely be able to do better than this, but I guess that might be a change we can do another time;

[mattico]

There's the embedded-time crate which is being integrated into rtic maybe. We could use it as well.

[richardeoin]

+1 on looking into the embedded-time crate. But for now this PR is useful on its own.

bors r+

[bors]

Build succeeded:

• ci (1.43.0, stm32h743)
• ci (1.43.0, stm32h743v)
• ci (1.43.0, stm32h747cm7)
• ci (1.43.0, stm32h753)
• ci (1.43.0, stm32h753v)
• ci (stable, stm32h743)
• ci (stable, stm32h743v)
• ci (stable, stm32h747cm7)
• ci (stable, stm32h753)
• ci (stable, stm32h753v)
• Rustfmt