dts: atmel sam: Document what rxpo and txpo mean.

The meaning of txpo is was a bit confusing, as it's an enumeration
rather than a pad number. This confusion extended to the atsamd21_xpro
board using the wrong pins.

This commit adds ASCII-art tables that explain the meaning of rxpo
and txpo on different platforms.

Signed-off-by: Tom Rothamel <tom@rothamel.us>