ARM: dts: n900: Use linux input defines instead hardcoded constants

This makes DTS structure more readable.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Acked-By: Sebastian Reichel <sre@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Tony Lindgren <tony@atomide.com>