xeon/spr: Set ACPI CPU string for 12bit

On platforms with more than 255 cores the ACPI CPU string
would overflow and generate duplicates. Fix that by changing
the string to hex and use 3 digits.

Test:
Able to boot without ACPI errors on IBM/SBP1 which has
384 actives cores.

Change-Id: I1887928da0c049c27e2ec129f49051b24048b33b
Signed-off-by: Naresh Solanki <Naresh.Solanki@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73368
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Jonathan Zhang <jon.zhixiong.zhang@gmail.com>

src/soc/intel/xeon_sp/spr/Kconfig

@@ -22,6 +22,10 @@ config MAX_CPUS
 	int
 	default 255
 
+config ACPI_CPU_STRING
+	string
+	default "\\_SB.C%03X"
+
 config MAX_SOCKET_UPD
 	int
 	default 2