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.

While at it, slightly modify the headers to reflect the usage of APIs.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Link: https://lore.kernel.org/r/20211216151227.58687-1-andriy.shevchenko@linux.intel.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>