mfd: mt6397: Use PLATFORM_DEVID_NONE macro instead of -1

Use the correct macro when adding the MFD devices instead of using
directly '-1' value.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>