MAINTAINERS: Add myself as the I3C subsystem maintainer

Create an entry for the I3C subsystem and mark it as maintained by me.
There's no official git repository, patchwork instance, mailing list or
website yet, but this will be added after the subsystem has been
accepted.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>