mhi: core: add support for bi-direction or directionless channels

MHI channels are not always unidirectional. Some offload
channels are either bi-directional or has no direction
associate with it, add support for such channels.

CRs-Fixed: 2229699
Change-Id: I6cb3b879eb3d90e9bbf8704805a16f1b43306e0d
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>