leds: triggers: add invert to heartbeat

This patch adds possibility to invert heartbeat blinking.
The inverted LED is more time ON then OFF. It's because it looks
better when the heartbeat LED is next to other LED which is most
time ON. The invert value is exported same way via sysfs in file
invert like oneshot. I get inspiration from this trigger.

Signed-off-by: Jiri Prchal <jiri.prchal@aksignal.cz>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>