acpigen: Add and use acpigen_write_method.

The sequence of bytes to create a method is used several times in codebase.
Put it into a function with logical arguments rather than duplicating magic
bytes everywhere.

Change-Id: I0e55d8dc7d5e8e92a521c7a83117c470d0614008
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/7347
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

src/cpu/intel/model_206ax/acpi.c

@@ -106,10 +106,7 @@ static void generate_C_state_entries(void)
 	if (!cpu || !cpu->cstates)
 		return;
 
-	acpigen_emit_byte(0x14);		/* MethodOp */
-	acpigen_write_len_f();		/* PkgLength */
-	acpigen_emit_namestring("_CST");
-	acpigen_emit_byte(0x00);		/* No Arguments */
+	acpigen_write_method("_CST", 0);
 
 	/* If running on AC power */
 	acpigen_emit_byte(0xa0);		/* IfOp */