platform/chrome: Add Chrome OS keyboard backlight LEDs support

This is a driver for ACPI-based keyboard backlight LEDs found on
Chromebooks. The driver locates \\_SB.KBLT ACPI device and exports
backlight as "chromeos::kbd_backlight" LED class device in sysfs.

Signed-off-by: Simon Que <sque@chromium.org>
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Tested-by: Evan McClain <aeroevan@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Olof Johansson <olof@lixom.net>