leds: convert IDE trigger to common disk trigger

This patch converts the IDE specific LED trigger to a generic disk
activity LED trigger. The libata core is now a trigger source just
like before the IDE disk driver. It's merely a replacement of the
string ide by disk.

The patch is taken from http://dev.gentoo.org/~josejx/ata.patch

 and is
widely used by any ibook/powerbook owners with great satisfaction.
Likewise, it is very often used successfully on different ARM platforms.

Unlike the original patch, the existing 'ide-disk' trigger is still
available for backward compatibility. That reduce the amount of patches
in affected device trees out of the mainline kernel. For further
development, the new name 'disk-activity' should be used.

Cc: Joseph Jezak <josejx@gentoo.org>
Cc: Jörg Sommer <joerg@alea.gnuu.de>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Stephan Linz <linz@li-pro.net>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>