powerpc/52xx: Get rid of of_node assignment

Let GPIO library assign of_node from the parent device.
This allows to move GPIO library and drivers to use fwnode
APIs instead of being stuck with OF-only interfaces.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220507100147.5802-3-andriy.shevchenko@linux.intel.com