nb/intel/ironlake/acpi: Replace Index(a, b) with ASL 2.0 syntax

Change-Id: I6af7131e151700569d50e8bc42bfaeb7a58fa7d3
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/71507
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/northbridge/intel/ironlake/acpi/hostbridge.asl

@@ -98,7 +98,7 @@ Device (MCHC)
 
 		While (Local0 < Local1) {
 			/* Store _PSS entry Control value to Local2 */
-			Local2 = DeRefOf (Index (DeRefOf (Index (\_SB.CP00._PSS, Local0)), 4)) >> 8
+			Local2 = DeRefOf (DeRefOf (\_SB.CP00._PSS[Local0])[4]) >> 8
 			If (Local2 == Arg0) {
 				Return (Local0 - 1)
 			}