spi: mpc5xxx-psc: Consistently use device property APIs

Instead of calling the OF APIs mixed with device property APIs,
just switch to use the latter everywhere.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230306183115.87314-6-andriy.shevchenko@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>