HID: apple: Add fn mapping for MacBook Pros with Touch Bar

This patch adds the Fn mapping for keyboards on certain T2 Macs.

[jkosina@suse.cz: rebase on top of apple_setup_input() refactoring]
Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>