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>
diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index f182613..1e8287b 100644
--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -92,7 +92,7 @@
  *
  * To skip this limit, boot/load with a large max_cstate limit.
  */
-static int set_max_cstate(struct dmi_system_id *id)
+static int set_max_cstate(const struct dmi_system_id *id)
 {
 	if (max_cstate > ACPI_PROCESSOR_MAX_POWER)
 		return 0;