sandybridge/igd: Add brightness register descriptions

Needed for brightness control for Lenovo X230

Change-Id: Ib6d127d2e050671dd402c31af06ff4726f65156c
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/4618
Tested-by: build bot (Jenkins)
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

src/northbridge/intel/sandybridge/acpi/igd.asl

@@ -23,6 +23,23 @@ Device (GFX0)
 {
 	Name (_ADR, 0x00020000)
 
+	OperationRegion (GFXC, PCI_Config, 0x00, 0x0100)
+	Field (GFXC, DWordAcc, NoLock, Preserve)
+	{
+		Offset (0x10),
+		BAR0, 64
+	}
+
+	OperationRegion (GFRG, SystemMemory, And(BAR0, 0xfffffffffffffff0), 0x400000)
+	Field (GFRG, DWordAcc, NoLock, Preserve)
+	{
+		Offset (0x48254),
+			BCLV, 16,
+		Offset (0xc8250),
+			CR1, 32,
+			CR2, 32
+	}
+
 	/* Display Output Switching */
 	Method (_DOS, 1)
 	{