backlight: add backlight type

backlight: add backlight type

There may be multiple ways of controlling the backlight on a given
machine.  Allow drivers to expose the type of interface they are
providing, making it possible for userspace to make appropriate policy
decisions.

Signed-off-by: Matthew Garrett <[email protected]>
Cc: Richard Purdie <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Ben Skeggs <[email protected]>
Cc: Zhang Rui <[email protected]>
Cc: Len Brown <[email protected]>
Cc: Jesse Barnes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>