soc/amd/picasso/cpu: make sure that MAX_CPUS isn't overridden

Trying to limit the number of available cores by setting the MAX_CPUS
Kconfig option to a lower value than the SoC's default might result in
cores being enabled in the FSP-S, but not fully initialized in coreboot
which will cause some malfunction. Add a static assert to make sure
that this option isn't changed from the default. To limit the maximum
number of cores, use the downcore_mode and disable_smt devicetree
settings instead.

TEST=Build fails if MAX_CPUS isn't the expected default.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I3cfe09f8bb89a2154d37a37398df982828c824f9
Reviewed-on: https://review.coreboot.org/c/coreboot/+/52611
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/picasso/cpu.c

@@ -3,6 +3,7 @@
 #include <amdblocks/cpu.h>
 #include <amdblocks/reset.h>
 #include <amdblocks/smm.h>
+#include <assert.h>
 #include <cpu/cpu.h>
 #include <cpu/x86/mp.h>
 #include <cpu/x86/mtrr.h>
@@ -18,6 +19,9 @@
 #include <console/console.h>
 #include <cpu/amd/microcode.h>
 
+_Static_assert(CONFIG_MAX_CPUS == 8, "Do not override MAX_CPUS. To reduce the number of "
+	"available cores, use the downcore_mode and disable_smt devicetree settings instead.");
+
 /*
  * MP and SMM loading initialization.
  */