Stabilizing CpuClock

[MabezDev]

As part of #2499

We're discussing how to stabilize the minimum amount. Or whether this should be stabilized at all.

Stabilize default and max

For each chip we only stabilize the lowest and highest clock speed, and expose default and max methods.

Stabilize CpuClock as is?

Whilst this enum covers every chip we currently support, could run into an issue with future chips, mostly around dynamic frequencies - if a future chip supports that. Given that it's a new chip, we can introduce a different API for this, so I don't see this being an issue.

Don't stabilize it

This option is quite self explanatory, we just mark it as #[unstable], and in esp_hal::Config and come back to stabilize this at a later date. Clock setting is only really required for use with esp-wifi, which itself is currently unstable.

[bjoernQ]

I vote for Stabilize CpuClock as is

[MabezDev]

Let's stabilize this as is. We can mark it as #[non_exaustive] and relax it later on, as we're doing with the clock source enums.