powerpc/5200: dts: refactor dts files

This patch creates mpc5200b.dtsi containing the information for the MPC5200b
SoC then modifies all of the dts files for MPC5200b based systems to use
mpc5200b.dtsi.

Signed-off-by: John Bonesio <bones@secretlab.ca>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>