input: apple: Add support for Apple SPI keyboard

This driver adds support for the keyboard on Apple Silicon laptops.
The controller for this keyboard sits on an SPI bus and uses an
Apple-specific HID over SPI protocol. The packets sent by this
controller for key presses and key releases are fairly simple and
are decoded directly by the code in this driver and converted into
standard Linux keycodes. The same controller handles the touchpad
found on these laptops.  Packets for touchpad events are simply
ignored.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested on: Macbook Air M1
Tested-by: Simon Glass <sjg@chromium.org>