HID: hid-appletb-bl: add driver for the backlight of Apple Touch Bars

This commit adds a driver for the backlight of Apple Touch Bars on x86
Macs. Note that currently only T2 Macs are supported.

This driver is based on previous work done by Ronald Tschalär
<ronald@innovation.ch>.

Signed-off-by: Kerem Karabay <kekrby@gmail.com>
Co-developed-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>