vendorcode/intel/fsp/fsp2_0/cpx_sp: Set correct stack number for IOU3

PSTACK2 (IOU3) should be stack number 4, mainboard uses stack number as
the index to access the bus number array read by get_stack_busnos().
Without the fix it would get the wrong bus number (0xb1).

Tested=On OCP Delta Lake, dmidecode -t 9 to verify slots bus number on
IOU3 are correct (0xb2).

Change-Id: I1c9e49bbc9a00de82d1fc67b3b4ed47e03eacdda
Signed-off-by: Johnny Lin <johnny_lin@wiwynn.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45022
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Zhang <jonzhang@fb.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/vendorcode/intel/fsp/fsp2_0/cooperlake_sp/hob_iiouds.h

@@ -111,13 +111,20 @@ typedef enum {
 } PCIE_PORTS;
 
 /**
- IIO Stacks
- **/
+ * IIO Stacks
+ *
+ * Ports    Stack	Stack(HOB)	IioConfigIou
+ * =================================================
+ * 0        CSTACK	stack 0		IOU0
+ * 1A..1D   PSTACK0	stack 1		IOU1
+ * 2A..2D   PSTACK1	stack 2		IOU2
+ * 3A..3D   PSTACK2	stack 4		IOU3
+ */
 typedef enum {
 	CSTACK = 0,
 	PSTACK0,
 	PSTACK1,
-	PSTACK2,
+	PSTACK2 = 4,
 	MAX_STACKS
 } IIO_STACKS;