x86: Add cpu code for x86_64

There is not much needed at present, but set up a separate directory to put
this code as it grows.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/x86/cpu/Makefile b/arch/x86/cpu/Makefile
index 41ad481..7f89ff0 100644
--- a/arch/x86/cpu/Makefile
+++ b/arch/x86/cpu/Makefile
@@ -38,6 +38,8 @@
 obj-$(CONFIG_SMP) += sipi_vector.o
 obj-y += turbo.o
 
-ifeq ($(CONFIG_$(SPL_)X86_64),)
+ifeq ($(CONFIG_$(SPL_)X86_64),y)
+obj-y += x86_64/
+else
 obj-y += i386/
 endif