[POWERPC] mpc512x: Basic platform support

512x is very similar to 83xx and most
of this is patterned after code from 83xx.

New platform:
    changed:
	arch/powerpc/Kconfig
	arch/powerpc/platforms/Kconfig
	arch/powerpc/platforms/Kconfig.cputype
	arch/powerpc/platforms/Makefile
    new:
	arch/powerpc/platforms/512x/*
	include/asm-powerpc/mpc512x.h

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