MAINTAINERS: Add an IRC channel for the I3C subsystem

The #linux-i3c chan has been created on freenode to discuss I3C related
topics.
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>

MAINTAINERS

@@ -7161,6 +7161,7 @@ F:	drivers/i2c/i2c-stub.c
 I3C SUBSYSTEM
 M:	Boris Brezillon <bbrezillon@kernel.org>
 L:	linux-i3c@lists.infradead.org
+C:	irc://chat.freenode.net/linux-i3c
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/i3c/linux.git
 S:	Maintained
 F:	Documentation/ABI/testing/sysfs-bus-i3c