backlight: add support for Pandora backlight

Add support for pandora (openpandora.org) backlight.

It might look like all this could be done using pwm_bl.c instead, but
there is a need of special programming sequence when turning on the LED
driver chip or else it will misbehave.  Doing this using pwm_bl.c would
require to use some register programming and pwm functions from platform
code, and ARM maintainers are allergic to driver-like code in /arch/arm
nowadays.  The PMIC PWM driver is currently missing too, so pwm_bl.c
can't be used anyway.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>