x86: Add ivybridge directory to Makefile

It is now required to add subdirectories in the x86 cpu Makefile. Add this
to fix a build breakage for chromebook_link.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/cpu/Makefile b/arch/x86/cpu/Makefile
index 2b9e9b9..818969f 100644
--- a/arch/x86/cpu/Makefile
+++ b/arch/x86/cpu/Makefile
@@ -13,4 +13,6 @@
 obj-y	+= interrupts.o cpu.o call64.o
 
 obj-$(CONFIG_SYS_COREBOOT) += coreboot/
+obj-$(CONFIG_NORTHBRIDGE_INTEL_SANDYBRIDGE) += ivybridge/
+obj-$(CONFIG_NORTHBRIDGE_INTEL_IVYBRIDGE) += ivybridge/
 obj-$(CONFIG_PCI) += pci.o