arch/x86/ioapic: Select IOAPIC with SMP

For coreboot proper, I/O APIC programming is not really required,
except for the APIC ID field. We generally do not guard the related
set_ioapic_id() or setup_ioapic() calls with CONFIG(IOAPIC).
In practice it's something one cannot leave unselected, but maintain
the Kconfig for the time being.

Change-Id: I6e83efafcf6e81d1dfd433fab1e89024d984cc1f
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55291
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/Kconfig

@@ -717,6 +717,7 @@ config PCI_IO_CFG_EXT
 
 config IOAPIC
 	bool
+	default y if SMP
 	default n
 
 config USE_WATCHDOG_ON_BOOT