Extract inner modules of gpio module into their own files

[jessebraham]

Our GPIO driver is getting quite large and complex, and each of these modules have sufficient complexity to warrant their own files, IMO, so I have extracted the modules.

There is one breaking change here, in that I changed the lp_gpio module to lp_io. This was done for two reasons: to be consistent with the peripheral name (LP_IO) and to be consistent with the rtc_io module in the same driver. I can revert this change if needed.

[SergioGasquez]

Although I'm pretty new with the peripheral, changes LGTM and the renaming also makes sense to me. Just left 2 minors docstrings suggestions that you can ignore if you want.

[jessebraham]

Good eye @SergioGasquez, thanks! Doc comment has been added.

I just realized there are a number of public items in the gpio module which still required doc comments as well; I am happy to take care of these too, but in the interest of getting this merged (also I have some other stuff on the go) I will probably break my new rule and do this in a separate PR 😬

[bjoernQ]

LGTM - I guess the amount of code in each module justifies having them separate

[JurajSadel]

LGTM, thanks!

[MabezDev]

LGTM, thanks!