slimbus: Provide separate APIs to connect source and sink ports

Source and sink ports for a channel may be connected from different
clients.
Multiple sink ports and 1 source port can be connected to a channel.
Ports are freed/zeroed when channel is removed.

Change-Id: I165bb9db4fb43a9a884ea14ffed08f8d11b65c39
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>