drvs/intel/gma/acpi: Add methods to use MBOX3

* Add Mailbox 3 driver
* Request brightness change through Mailbox 3
* Return Ones on error or if unsupported
* Mark existing code as legacy
  (still required if no GMA driver is running)
* Call legacy code if Mailbox 3 is unsupported, on error or
  if gma driver isn't running

Tested on Lenovo T430:
* Brightness control still works
* Brightness is the same on S3 resume

Change-Id: I51554c819148336b204d28972cbf775a10c3fb8a
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/27711
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Nathaniel Roach <nroach44@gmail.com>
Reviewed-by: Nicola Corna <nicola@corna.info>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
diff --git a/src/drivers/intel/gma/acpi/non-pch.asl b/src/drivers/intel/gma/acpi/non-pch.asl
index ef8f3ff..c35d251 100644
--- a/src/drivers/intel/gma/acpi/non-pch.asl
+++ b/src/drivers/intel/gma/acpi/non-pch.asl
@@ -22,7 +22,11 @@
 	Field (GFXC, DWordAcc, NoLock, Preserve)
 	{
 		Offset (0x10),
-		BAR0, 64
+		BAR0, 64,
+		Offset (0xe4),
+		ASLE, 32,
+		Offset (0xfc),
+		ASLS, 32,
 	}
 
 	OperationRegion (GFRG, SystemMemory, And (BAR0, 0xfffffffffffffff0), 0x400000)