platform/surface: Add Surface Aggregator user-space interface

Add a misc-device providing user-space access to the Surface Aggregator
EC, mainly intended for debugging, testing, and reverse-engineering.
This interface gives user-space applications the ability to send
requests to the EC and receive the corresponding responses.

The device-file is managed by a pseudo platform-device and corresponding
driver to avoid dependence on the dedicated bus, allowing it to be
loaded in a minimal configuration.

A python library and scripts to access this device can be found at [1].

[1]: https://github.com/linux-surface/surface-aggregator-module/tree/master/scripts/ssam

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20201221183959.1186143-9-luzmaximilian@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>