Add platform specific keywords.txt

Since arduino/Arduino@3f1b23b 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 arduino/Arduino#3534

Author: per1234

keywords.txt

@@ -0,0 +1 @@
+INPUT_PULLDOWN	LITERAL1	Constants	RESERVED_WORD_2