[POWERPC] Separate MPC52xx PSC FIFO registers from rest of PSC

This is in preparation for the addition of MPC512x
PSC support.  The main difference in the 512x is
in the fifo registers.

Signed-off-by: John Rigby <jrigby@freescale.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>