powerpc/corenet64: compile with CONFIG_E{5,6}500_CPU well

If CONFIG_ALTIVEC is enabled for CoreNet64, and if we also
select CONFIG_E{5,6}500_CPU this may introduce -mcpu=e500mc64
into $CFLAGS. But Altivec option not allowed with e500mc64,
then some compiling errors occur like this:

      CC      arch/powerpc/lib/xor_vmx.o
    arch/powerpc/lib/xor_vmx.c:1:0: error: AltiVec not supported in this target
    make[1]: *** [arch/powerpc/lib/xor_vmx.o] Error 1
    make: *** [arch/powerpc/lib] Error 2

So we should restrict e500mc64 in altivec scenario.

Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>