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

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

Change-Id: If848d391e5ec33ebfb08515414739dbdd5011e08
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66249
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Sean Rhodes <sean@starlabs.systems>
Reviewed-by: Sridhar Siricilla <sridhar.siricilla@intel.com>

src/arch/x86/acpi/debug.asl

@@ -77,9 +77,9 @@ Method(DBGN, 1)
 {
 	and(Arg0, 0x0f, Local0)
 	if (LLess(Local0, 10)) {
-		add(Local0, 0x30, Local0)
+		Local0 += 0x30
 	} else {
-		add(Local0, 0x37, Local0)
+		Local0 += 0x37
 	}
 	OUTC(Local0)
 }