i3c: Add support for bus enumeration & notification

This allows other drivers to be notified when new i3c busses are
attached, referring to a whole i3c bus as opposed to individual
devices.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>