backlight: add an AS3711 PMIC backlight driver

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>