ARM: dts: n900: cleanup

Fix GPIO comment to be consistent with rest of file and add comment what
tpa6130 is.

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