powerpc/mpc512x: move common code to shared.c file

- implement all of the init, init early, and setup arch routines in the
  shared source file for the MPC512x PowerPC platform, and make all
  MPC512x based boards (ADS, PDM, generic) use those common routines
- remove declarations from header files for routines which aren't
  referenced from external callers any longer

this modification concentrates knowledge about the optional FSL DIU
support in one spot within the shared code, and makes all boards benefit
transparently from future improvements in the shared platform code

the change does not modify any behaviour but preserves all code paths

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>