cpu/qemu-x86: Increase MAX_CPUS to have actual AP init

CONFIG_MAX_CPUS=4 is the maximum supported with SMM_ASEG.

TESTED: on q35 and i440fx -smp 4/32.

Change-Id: I696856870e34e7a7ad580bc83c6b38f1dfb4511d
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48209
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/cpu/qemu-x86/Kconfig

@@ -28,6 +28,11 @@ config CPU_QEMU_X86_ASEG_SMM
 
 endchoice
 
+config MAX_CPUS
+	int
+	default 4 if SMM_ASEG
+	default 32
+
 config CPU_QEMU_X86_64
 	bool "Experimental 64bit support"
 	select ARCH_ALL_STAGES_X86_64