Since https://github.com/arduino/Arduino/commit/3f1b23bf6123e680ed1ddc85d5f39b3555aade0b this will cause keywords in this file to be highlighted only when a board from this platform is selected.

Currently only contains INPUT_PULLDOWN. Style copied from the INPUT_PULLUP entry at https://github.com/arduino/Arduino/blob/1.8.3/build/shared/lib/keywords.txt#L11.

I don't know whether there are other platform specific keywords for Arduino SAMD Boards. I'm happy to add any others on request.

Closes https://github.com/arduino/Arduino/issues/3534