video/lcd: document sysfs interface

Date and KernelVersion may be wrong because the lcd
interface was introduced before git initial import.

Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Len Brown <len.brown@intel.com>

Documentation/ABI/testing/sysfs-class-lcd

+What:		/sys/class/lcd/<lcd>/lcd_power
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	Richard Purdie <rpurdie@rpsys.net>
+Description:
+		Control LCD power, values are FB_BLANK_* from fb.h
+		 - FB_BLANK_UNBLANK (0)   : power on.
+		 - FB_BLANK_POWERDOWN (4) : power off
+
+What:		/sys/class/lcd/<lcd>/contrast
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	Richard Purdie <rpurdie@rpsys.net>
+Description:
+		Current contrast of this LCD device. Value is between 0 and
+		/sys/class/lcd/<lcd>/max_contrast.
+
+What:		/sys/class/lcd/<lcd>/max_contrast
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	Richard Purdie <rpurdie@rpsys.net>
+Description:
+		Maximum contrast for this LCD device.