inteltool: update documentation

- manpage
 - usage message
 - new warning message if -S is used on an unsupported chipset

Change-Id: I1acaa5f4232b65244ec00fd22ec7460d9cc387f1
Signed-off-by: Stefan Tauner <stefan.tauner@gmx.at>
Reviewed-on: https://review.coreboot.org/14624
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

util/inteltool/ivy_memory.c

@@ -359,7 +359,7 @@ void ivybridge_dump_timings(const char *dump_spd_file)
 
 	printf("/* SPD matching current mode:  */\n");
 
-	FILE *dump_spd = 0;
+	FILE *dump_spd = NULL;
 
 	if (dump_spd_file) {
 		dump_spd = fopen (dump_spd_file, "wb");