backlight: add an AS3711 PMIC backlight driver

This is an initial commit of a backlight driver, using step-up DCDC
power supplies on AS3711 PMIC.  Only one mode has actually been tested,
several further modes have been implemented "dry," but disabled to avoid
accidental hardware damage.  Anyone wishing to use any of those modes
will have to modify the driver.

Tested on sh73a0-based kzm9g board.  Only one mode has been tested and
is enabled.  That mode copies the sample code from the manufacturer.
Deviations from that code proved to be fatal for the hardware...

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Magnus Damm <magnus.damm@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>