Similar to PWM, I2C can only be used on some pins. To automatically
generate this information per board, we need to add extra comments that
can then be interpreted by doc-gen for the tinygo.org website.
Right now it doesn't compile, with errors like the following:
# machine
/app/tinygo/src/machine/board_pico.go:7:13: undefined: GPIO0
/app/tinygo/src/machine/board_pico.go:8:13: undefined: GPIO1
/app/tinygo/src/machine/board_pico.go:9:13: undefined: GPIO2
/app/tinygo/src/machine/board_pico.go:10:13: undefined: GPIO3
[...etc...]
This patch should fix that.