backlight: pcf50633: Register a pcf50633-backlight device in pcf50633 core driver.

Register a device newly added pcf50633-backlight driver as a child device in
the pcf50633 core driver.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>