backlight: add LM3533 backlight driver

backlight: add LM3533 backlight driver

Add sub-driver for the backlights on National Semiconductor / TI LM3533
lighting power chips.

The chip provides 256 brightness levels and ambient-light-sensor and pwm
input control.

[[email protected]: fix warning]
[[email protected]: fix the type of `mode']
Signed-off-by: Johan Hovold <[email protected]>
Cc: Richard Purdie <[email protected]>
Cc: Rob Landley <[email protected]>
Cc: Samuel Ortiz <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Florian Tobias Schandinat <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Mark Brown <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>