cpu/x86/smm/smm.ld: Assert that CONFIG_MAX_CPUS <= 4

The SMM_ASEG code only supports up to 4 CPUs, so assert this at
buildtime.

Change-Id: I8ec803cd1b76f17f4dccd5c573179d542d54c277
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44322
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/cpu/x86/smm/smm.ld

@@ -2,6 +2,9 @@
 
 /* Maximum number of CPUs/cores */
 CPUS = 4;
+
+_ = ASSERT(CPUS >= CONFIG_MAX_CPUS, "The ASEG SMM code only supports up to 4 CPUS");
+
 ENTRY(smm_handler_start);
 
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