mfd: Revert "mfd: Always assign of_node in mfd_add_device()"

This reverts commit 68044bee13770918e0b28dd44aa98c889ec7558f.

We've had confirmed reports of this patch causing unforeseen issues
with existing MFD users. It has been agreed by the original author
and myself that reversion is the best solution.

Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>