i3c: Add a modalias sysfs attribute

Create a modalias sysfs attribute for i3c devices.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Vitor Soares <vitor.soares@synopsys.com>
Link: https://lore.kernel.org/linux-i3c/a90f64f830128cd12762153de7828b775574c156.1582796652.git.vitor.soares@synopsys.com