powerpc/pseries: re-use code from of_helpers module

The derive_parent() has similar semantics to what we have in newly introduced
of_helpers module. The replacement reduces code base and propagates the actual
error code to the caller.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>