diag: Modularise diagfwd transport bridge registration

Modify the diagfwd transport bridge registration such that the
choice between MHI/HSIC is made from diagfwd_bridge rather than
diagchar_core driver, thus improving the readability and
extensibility in case a new transport comes up in future.

Change-Id: Ia4697b842a2a4dd87789a32a246e92cf39562048
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>