Input: add driver for power button on Dell Wyse 3020

This adds support for the power button attached to the Embedded Controller
on a Dell Wyse 3020 "Ariel" board.

The Embedded Controller's SPI interface is actually capable sending and
receiving the PS/2 keyboard and mouse protocol data, which looks like
a good fit for a serio driver. Howerver, I don't know of any machines where
this is actually used.

My board only has a single power button and no way to connect an actual
keyboard or a mouse. Using the atkbd driver with serio would be an overkill
and would be inconvenient for the userspace. Therefore this driver
registers an input device that is only capable of reporting the power
button presses and releases.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20201201083533.1724287-3-lkundrak@v3.sk
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>