S3: Fix LowPowerInput out of bounds panic

[JurajSadel]

LowPowerInput and LowPowerOutputOpenDrain now store pin and they "delegate" rtcio_ functionality.
closes #3399

[bugadani]

My only problem is that now we have to document this in the changelog and migration guide, but the API is:

• generic (you could use AnyPin, I think)
• does not follow the structure of GPIO drivers (or other drivers, even)
• currently this is only meant for configuring pins before passing them to LP cores - there is no way to set outputs, read inputs, which still could be useful in HP core code
• code in lp_io is unchanged