drm/nouveau/drm/nouveau: add a LED driver for the NVIDIA logo

We received a donation of a Titan which has this useless feature
allowing users to control the brightness of the LED behind the
logo of NVIDIA. In the true spirit of open source, let's expose
that to the users of very expensive cards!

This patch hooks up this LED/PWM to the LED subsystem which allows
blinking it in sync with cpu/disk/network/whatever activity (heartbeat
is quite nice!). Users may also implement some breathing effect or
morse code support in the userspace if they feel like it.

v2:
 - surround the use of the LED framework with ifdef CONFIG_LEDS_CLASS

v3:
 - avoid using ifdefs everywhere, follow the recommendations of
   /doc/Documentation/CodingStyle. Suggested by Emil Velikov.

v4 (Ben):
 - squashed series of fixes from ml

Signed-off-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>