drivers/firmware: const-ify DMI API and internals

Three main sets of changes:

1) dmi_get_system_info() return value should have been marked const,
   since callers should not be changing that data.

2) const-ify DMI internals, since DMI firmware tables should,
   whenever possible, be marked const to ensure we never ever write to
   that data area.

3) const-ify DMI API, to enable marking tables const where possible
   in low-level drivers.

And if we're really lucky, this might enable some additional
optimizations on the part of the compiler.

The bulk of the changes are #2 and #3, which are interrelated.  #1 could
have been a separate patch, but it was so small compared to the others,
it was easier to roll it into this changeset.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/misc/thinkpad_acpi.c

@@ -4448,7 +4448,7 @@ static void ibm_exit(struct ibm_struct *ibm)
 
 static void __init get_thinkpad_model_data(struct thinkpad_id_data *tp)
 {
-	struct dmi_device *dev = NULL;
+	const struct dmi_device *dev = NULL;
 	char ec_fw_string[18];
 
 	if (!tp)