ec/google: Detect keyboard backlight at runtime

This enables adding the backlight driver to boards that may or may not
come with a keyboard backlight function.
It's the responsibility of the EC to report if that feature exists, but
that's not a big extra burden given that it already keeps track of
everything else related to the backlight.

BUG=b:64705535
BRANCH=none
CQ-DEPEND=CL:620595
TEST=configured KBLE manually and noticed the presence/absence of
/sys/devices/platform/GOOG0002:00/ on a Chrome OS Linux kernel,
corresponding to the value reported by the EC.

Change-Id: Idc36bfaa6e69581ba19b52d37af6956f63cfdb8f
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/21099
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/ec/google/chromeec/acpi/ec.asl

@@ -54,6 +54,8 @@ Device (EC0)
 		PATC, 8,	// Programmable Auxiliary Trip Commit
 		CHGL, 8,	// Charger Current Limit
 		TBMD, 1,	// Tablet mode
+		Offset (0x0a),
+		KBLE, 1,	// Keyboard Backlight exists
 	}
 
 #if IS_ENABLED(CONFIG_EC_GOOGLE_CHROMEEC_ACPI_MEMMAP)

src/ec/google/chromeec/acpi/keyboard_backlight.asl

@@ -23,6 +23,20 @@ Scope (\_SB)
 		Name (_HID, "GOOG0002")
 		Name (_UID, 1)
 
+		/* Ask EC if we even have a backlight
+		 * Return 0xf (present, enabled, show in UI, functioning) or 0
+		 *
+		 * With older EC codebases that don't support the Device
+		 * Features bitfield, this reports the keyboard backlight as
+		 * enabled since reads to undefined addresses in EC address
+		 * space return 0xff and so KBLE will be 1.
+		 */
+		Method (_STA, 0, NotSerialized)
+		{
+			Multiply (0xf, \_SB.PCI0.LPCB.EC0.KBLE, Local0)
+			Return (Local0)
+		}
+
 		/* Read current backlight value */
 		Method (KBQC, 0, NotSerialized)
 		{