[ARM] 5045/1: magician: use the pwm_bl driver for the LCD backlight

magician has a GPIO that modifies the brightness level additionally to
the PWM duty value. This patch makes use of the pwm_bl notify callback
to present userspace with a single brightness scale.
This gets rid of the pxa_set_cken calls and direct PWM register access.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>