dt-bindings: i3c: Document core bindings

A new I3C subsystem has been added and a generic description has been
created to represent the I3C bus and the devices connected on it.

Document this generic representation.

Change-Id: Ie245c4d5b264375c6ab6a80abe2a4777dc2af4cd
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Git-commit: 54820b4a6627e87afc0425c8b4ce338d3dbdbb80
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Alok Chauhan <alokc@codeaurora.org>