HID: hid-appletb-kbd: add driver for the keyboard mode of Apple Touch Bars

The Touch Bars found on x86 Macs support two USB configurations: one
where the device presents itself as a HID keyboard and can display
predefined sets of keys, and one where the operating system has full
control over what is displayed. This commit adds a driver for the
display functionality of the first configuration.

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>