soc/intel/cnl: add Kconfig values for GMA backlight registers

Add the right register values for backlight control to CNL's Kconfig.
To make iasl happy about the reversed register order, split the field.

Change-Id: I05a06cc42397c202df9c9a1ebc72fb10da3b10ec
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48772
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/drivers/intel/gma/acpi/gma.asl

@@ -20,6 +20,9 @@ Device (GFX0)
 	{
 		Offset (CONFIG_INTEL_GMA_BCLV_OFFSET),
 		BCLV, CONFIG_INTEL_GMA_BCLV_WIDTH,
+	}
+	Field (GFRG, DWordAcc, NoLock, Preserve)
+	{
 		Offset (CONFIG_INTEL_GMA_BCLM_OFFSET),
 		BCLM, CONFIG_INTEL_GMA_BCLM_WIDTH
 	}