HID: SFH: Add documentation

Add Documentation/hid/amd-sfh-hid.rst file which was by mistake dropped by
me when applying the series.

Fixes: 302f0dad

 ("SFH: Add maintainers and documentation for AMD SFH based on HID framework")
Reported-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Originally-by: Sandeep Singh <Sandeep.Singh@amd.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>