[POWERPC] mpc5200 device tree bindings refinement

Much needed refinement of mpc5200 device tree binding specifications.

Short list:
- drop mpc52xx designator; only two supported chips exist, 5200 and 5200b.
  It's premature to refer to them as '52xx'.
- Specify optional 'model' and 'revision' properties in the soc5200 node
- Specify reqiured 'cell-index' property to identify between multiple SOC
  devices of the same type.  (Useful for arbitrating shared register access)
- Specify optional 'port-number' property for adjusting the logical serial
  port assignments.
- Specify optional 'has-wdt' property for gpt0 node.
- Add system-frequency property to soc5200 node

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>