sony_acpi: Add backlight support to the sony_acpi v2

Enable the sony_acpi driver to use the backlight subsysyem for adjusting
the monitor brightness.  Old way of changing the brightness will be still
available for compatibility with existing tools.

Signed-off-by: Alessandro Guido <alessandro.guido@gmail.com>
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>