unicore32: use the new LED disk activity trigger

- platform: rename 'ide-disk' to 'disk-activity'
- defconfig: rename 'LEDS_TRIGGER_IDE_DISK' to 'LEDS_TRIGGER_DISK'

Signed-off-by: Stephan Linz <linz@li-pro.net>
Acked-by: GUAN Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>