mpc512x: fix build issues

Commit 800eb096 "POST cleanup." removed file
arch/powerpc/cpu/mpc512x/common.c but failed to remove the reference
to it from arch/powerpc/cpu/mpc512x/Makefile which causes somewhat
obscure build errors:

make[1]: *** No rule to make target `/work/wd/tmp-ppc/arch/powerpc/cpu/mpc512x/.depend', needed by `_depend'.  Stop.

Fix these.

Signed-off-by: Wolfgang Denk <wd@denx.de>