platform/x86: wmi: Add device specific documentation

Add a place for device-specific documentation of WMI drivers.
The first entry is documentation for the wmi-bmof driver, with
additional documentation being expected to follow.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20230424222939.208137-5-W_Armin@gmx.de
Signed-off-by: Hans de Goede <hdegoede@redhat.com>