[POWERPC] mpc52xx_psc_spi device driver must not touch port_config and cdm

It is dangerous for an mpc52xx device driver to modify the port_config
register.  If the driver is probed incorrectly, it will change the pin
IO configuration in ways which may not be compatible with the board.
port_config should be set up by the bootloader, or failing that, in
the platform setup code in arch/powerpc/platforms/52xx.

Also, modifying CDM registers directly can cause a race condition with
other drivers.  Instead call a common routine to modify CDM settings.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Dragos Carp <dragos.carp@toptica.com>