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>

arch/x86/cpu/Makefile

@@ -13,4 +13,6 @@ obj-$(CONFIG_X86_RESET_VECTOR) += resetvec.o start16.o
 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