mhi: core: add support to start channels automatically

Add support to start MHI channels automatically after
finishing mhi device probe. This helps to simplify
clients's driver and remove duplicate code between
multiple clients.

CRs-Fixed: 2237688
Change-Id: Ie11d5f4327a9c8039d1caab5cb2b89cc19e59b82
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>