backlight: sky81452: Privatize platform data

The only way the platform data for the SKY81452 ever gets populated
is through the device tree.

The MFD device is bothered with this for no reason at all. Just
allocate the platform data in the driver and be happy.

Cc: Gyungoh Yoo <jack.yoo@skyworksinc.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>