feat: add support for Sparkfun ProMini 3.3V 8Mhz (sparkfun-promini-3v3)

[tippfehlr]

This PR adds support for the ProMini 3.3V (8Mhz) (sparkfun-promini-3v3). Apart from arduino-hal/src/clock.rs it is entirely copied from #435.

~~Because sparkfun-promini-3.3v is not a valid name, I left out the .3.~~ ~~Another option would be to rename both the 5V and the 3.3V version to 16Mhz and 8Mhz respectively, eg.~~ ~~- sparkfun-promini-16mhz~~ ~~- sparkfun-promini-8mhz~~

~~Both variants are fine in my opinion, however renaming 5v to 16mhz would be a breaking change.~~

Original issue: #425

[stappersg]

This comment provides a link to a similar merge request

[tippfehlr]

@stappersg You linked to this PR, not #445, which I assume you meant.

True, I didn’t check for existing PRs. However, because this is mergeable as is, I’ll leave it open. I also changed the name to sparkfun-promini-3v3 because that it’s more appropriate.

https://github.com/Rahix/avr-hal/pull/445#issuecomment-1815774078: As I understand this the ProMini 3.3V is an Arduino (328P) clocked down, meaning it is only appropriate to handle it as such.