ARM: pxa/raumfeld: adapt to upcoming hardware change

The backlight control is going to change back to PWM in the
upcoming Raumfeld Controller hardware revision.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Acked-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>