diag: Redesign diag bridge

Refactor diagfwd_bridge to make it scalable and to communicate
with the remote devices in a much cleaner fashion. Also make
changes to DCI to reflect the changes in the bridge.

Change-Id: Ica5a3552ec94c5543518698390c910518da04f3b
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>