haswell boards: Factor out MAX_CPUS

ULT only has 4 threads, but we are not changing it here to preserve
binary reproducibility.

Change-Id: I041c5dff2de514244f9c919c4c475cca979c34ce
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41842
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/intel/haswell/Kconfig

@@ -34,4 +34,8 @@ config IED_REGION_SIZE
 config SMM_RESERVED_SIZE
 	hex
 	default 0x100000
+
+config MAX_CPUS
+	int
+	default 8
 endif