platform/x86: Add System76 ACPI driver

Add System76 ACPI driver, which adds support for Fn-Fx key
combinations, keyboard backlight, and airplane mode LEDs
on System76 laptops running open source firmware.

Signed-off-by: Jeremy Soller <jeremy@system76.com>
Cc: platform-driver-x86@vger.kernel.org
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>