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/thermal.c b/drivers/acpi/thermal.c
index bc6d586..ad898e1 100644
--- a/drivers/acpi/thermal.c
+++ b/drivers/acpi/thermal.c
@@ -1360,7 +1360,7 @@
 }
 
 #ifdef CONFIG_DMI
-static int thermal_act(struct dmi_system_id *d) {
+static int thermal_act(const struct dmi_system_id *d) {
 
 	if (act == 0) {
 		printk(KERN_NOTICE "ACPI: %s detected: "
@@ -1369,14 +1369,14 @@
 	}
 	return 0;
 }
-static int thermal_nocrt(struct dmi_system_id *d) {
+static int thermal_nocrt(const struct dmi_system_id *d) {
 
 	printk(KERN_NOTICE "ACPI: %s detected: "
 		"disabling all critical thermal trip point actions.\n", d->ident);
 	nocrt = 1;
 	return 0;
 }
-static int thermal_tzp(struct dmi_system_id *d) {
+static int thermal_tzp(const struct dmi_system_id *d) {
 
 	if (tzp == 0) {
 		printk(KERN_NOTICE "ACPI: %s detected: "
@@ -1385,7 +1385,7 @@
 	}
 	return 0;
 }
-static int thermal_psv(struct dmi_system_id *d) {
+static int thermal_psv(const struct dmi_system_id *d) {
 
 	if (psv == 0) {
 		printk(KERN_NOTICE "ACPI: %s detected: "