gpio: Propagate firmware node from a parent device

When creating MFD platform devices the firmware node is left unset.
This, in particular, prevents GPIO library to use it for different
purposes. Propagate firmware node from the parent device and let
GPIO library do the right thing.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>