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

arch/powerpc/Makefile

@@ -128,7 +128,12 @@ CFLAGS-$(CONFIG_POWER5_CPU) += $(call cc-option,-mcpu=power5)
 CFLAGS-$(CONFIG_POWER6_CPU) += $(call cc-option,-mcpu=power6)
 CFLAGS-$(CONFIG_POWER7_CPU) += $(call cc-option,-mcpu=power7)
 
+# Altivec option not allowed with e500mc64 in GCC.
+ifeq ($(CONFIG_ALTIVEC),y)
+E5500_CPU := -mcpu=powerpc64
+else
 E5500_CPU := $(call cc-option,-mcpu=e500mc64,-mcpu=powerpc64)
+endif
 CFLAGS-$(CONFIG_E5500_CPU) += $(E5500_CPU)
 CFLAGS-$(CONFIG_E6500_CPU) += $(call cc-option,-mcpu=e6500,$(E5500_CPU))