[POWERPC] mpc512x: Factor out 5200 dependencies from 52xx psc driver

PSC devices are different between the mpc5200 and the mpc5121
this patch localizes the differences in preparation for adding mpc5121
support to the psc uart driver.
Signed-off-by: John Rigby <jrigby@freescale.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>