sony_acpi: Add backlight support to the sony_acpi

Make the sony_acpi use the backlight subsystem to adjust brightness value
instead of using the /proc/sony/brightness file.  (Other settings will
still have a /proc/sony/...  entry)

Signed-off-by: Alessandro Guido <alessandro.guido@gmail.com>
Cc: Stelian Pop <stelian@popies.net>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Len Brown <len.brown@intel.com>