mfd: Conditionally add WM831x backlight subdevice

The WM831x backlight driver requires at least the specification of the
current sink to use and a maximum current to allow them to function and
will actively interfere with other users of the regulators it uses if
misconfigured so only register the subdevice for it if this platform
data has been supplied.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>