Novena headers to linux GPIO mappings

For reference, mapping of "extra" pins on Novena internal serial headers to GPIO pin numbers.

The layout is like this:

|-- |  | d  u  | e  a  | b  r  | u  t  | g  4  <- pin 1/GND | | u  | a  | r  | t  | 3     <- pin 1/GND |