cpu/x86/mp_init.c: Make it work for !CONFIG_SMP

With very little changes this code can be used to initialize systems
without SMP. The linker will remove most of the code.

Change-Id: Ia0e8fdf8ed7bc2e0e4ff01be8d3e3c3cb837e6c7
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59692
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/cpu/x86/Kconfig

@@ -1,12 +1,13 @@
 config PARALLEL_MP
 	def_bool y
 	depends on !LEGACY_SMP_INIT
-	depends on SMP
 	select CPU_INFO_V2
 	help
 	 This option uses common MP infrastructure for bringing up APs
 	 in parallel. It additionally provides a more flexible mechanism
 	 for sequencing the steps of bringing up the APs.
+	 The code also works for just initialising the BSP in case there
+	 are no APs.
 
 config PARALLEL_MP_AP_WORK
 	def_bool n

src/cpu/x86/mp_init.c

@@ -593,6 +593,10 @@ static enum cb_err mp_init(struct bus *cpu_bus, struct mp_params *p)
 		return CB_ERR;
 	}
 
+	/* We just need to run things on the BSP */
+	if (!CONFIG(SMP))
+		return bsp_do_flight_plan(p);
+
 	/* Default to currently running CPU. */
 	num_cpus = allocate_cpu_devices(cpu_bus, p);