arch/x86/acpi: Replace Add(a,b,c) with ASL 2.0 syntax

Replace `Add (a, b, c)` with `c = a + b`.

Change-Id: I10a94406aa7e66eed4fec22f56b7a8802252e781
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/60495
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

src/arch/x86/acpi/globutil.asl

@@ -24,7 +24,7 @@ Method(SLEN, 1)
 
 Method(S2BF, 1, Serialized)
 {
-	Add(SLEN(Arg0), One, Local0)
+	Local0 = SLEN(Arg0) + 1
 	Name(BUFF, Buffer(Local0) {})
 	Store(Arg0, BUFF)
 	Return(BUFF)