mpc512x: fix build issues

Commit 800eb0964 "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>

arch/powerpc/cpu/mpc512x/Makefile

@@ -29,7 +29,6 @@ LIB	= $(obj)lib$(CPU).a
 START	= start.o
 COBJS-y	:= cpu.o
 COBJS-y	+= traps.o
-COBJS-y += common.o
 COBJS-y += cpu_init.o
 COBJS-y += fixed_sdram.o
 COBJS-y += i2c.o