HID: hid-appletb-kbd: add support for fn toggle between media and function mode

This patch adds support for the switching between the Media and Function
keys on the touchbar by pressing the Fn key on Apple Internal Keyboard.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>