powerpc: move mpc8xxx entry under arch/powerpc/cpu/

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/powerpc/cpu/Makefile b/arch/powerpc/cpu/Makefile
new file mode 100644
index 0000000..d630abe
--- /dev/null
+++ b/arch/powerpc/cpu/Makefile
@@ -0,0 +1,3 @@
+ifneq ($(filter mpc83xx mpc85xx mpc86xx,$(CPU)),)
+obj-y += mpc8xxx/
+endif