x86: coreboot: Allow building an SPL image

Make a few adjustments to allow us to build an SPL image for coreboot.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/x86/cpu/Makefile b/arch/x86/cpu/Makefile
index 307267a..ee0499f 100644
--- a/arch/x86/cpu/Makefile
+++ b/arch/x86/cpu/Makefile
@@ -54,9 +54,11 @@
 obj-$(CONFIG_INTEL_QUEENSBAY) += queensbay/
 obj-$(CONFIG_INTEL_TANGIER) += tangier/
 obj-$(CONFIG_APIC) += lapic.o ioapic.o
-obj-$(CONFIG_$(SPL_TPL_)X86_32BIT_INIT) += irq.o
 obj-$(CONFIG_$(SPL_TPL_)ACPI_GPE) += acpi_gpe.o
 obj-$(CONFIG_QFW) += qfw_cpu.o
+ifndef CONFIG_SYS_COREBOOT
+obj-$(CONFIG_$(SPL_TPL_)X86_32BIT_INIT) += irq.o
+endif
 ifndef CONFIG_$(SPL_)X86_64
 obj-$(CONFIG_SMP) += mp_init.o
 endif