soc/intel/xeon_sp/gnr: Enlarge MAX_CPUS to 512

GNR-AP supports up-to 128 cores/256 threads per socket. Enlarge
MAX_CPUS to 512 = 128*2*2 with 2 socket configuration considered.

Change-Id: I8dc46dcdd3ca1c3ddfa47fbb28912a2c6e4c46fa
Signed-off-by: Jincheng Li <jincheng.li@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84312
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/xeon_sp/gnr/Kconfig

@@ -30,7 +30,7 @@ config FSP_HEADER_PATH
 
 config MAX_CPUS
 	int
-	default 255
+	default 512
 
 config PCR_BASE_ADDRESS
 	hex