Support for Pico second serial port

[o-gent]

Hi!

To my dismay I found Serial2 didn't work when I started messing with the Pico.

This change adds the pin definition required for Serial 2 to GPIOs 8 & 9. I used these since the default definition for i2c is on 4&5.

I've tested this on my setup and successfully received data on Serial2.

[CLAassistant]

All committers have signed the CLA.

[o-gent]

Hi @facchinm, I see you've merged a bunch of the previous pull requests - do you see any issue with this proposal?