powerpc: use the new LED disk activity trigger

- dts: rename 'ide-disk' to 'disk-activity'
- defconfig: rename 'ADB_PMU_LED_IDE' to 'ADB_PMU_LED_DISK'

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