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: NDaniel Drake <dsd@laptop.org>
Acked-by: NJonathan Corbet <corbet@lwn.net>
Signed-off-by: NFlorian Tobias Schandinat <FlorianSchandinat@gmx.de>