viafb: General power management infrastructure

Multiple devices need S/R hooks (framebuffer, GPIO, camera).
Add infrastructure and convert existing framebuffer code to the new
model.

This patch should create no functional change.
Based on earlier work by Jonathan Corbet.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>