add backlight support through sysfs

Signed-off-by: Mattias Andrée <maandree@operamail.com>
m

Signed-off-by: Mattias Andrée <maandree@operamail.com>
info: doc backlight

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add backlight example

Signed-off-by: Mattias Andrée <maandree@operamail.com>