atmel_lcdfb: backlight control

On the sam9 EK boards, the LCD backlight is hooked up to a PWM output from
the LCD controller.  It's controlled by "contrast" registers though.

This patch lets boards declare that they have that kind of backlight
control.  The driver can then export this control, letting screenblank and
other operations actually take effect ...  reducing the typically
substantial power drain from the backlight.

Note that it's not fully cooked
  - doesn't force backlight off during system suspend
  - the "power" and "blank" events may not be done right
This should be easily added in the future.

[[email protected]: remove unneeded inline and rename functions]
Signed-off-by: David Brownell <[email protected]>
Signed-off-by: Nicolas Ferre <[email protected]>
Cc: Andrew Victor <[email protected]>
Cc: Russell King <[email protected]>
Cc: "Antonino A. Daplas" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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