i3c: Add sysfs ABI spec

Document sysfs files/directories/symlinks exposed by the I3C subsystem.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>