net: ipc_router: Move redundant definition to IPC Router Core

The helper function to load the default peripheral is defined redundantly
in multiple IPC Router transports.

Move the redundant definition of loading the default peripheral to the
IPC Router Core. Add an optional default peripheral property to enable
IPC Router to load and communicate with that peripheral.

Change-Id: I78cfb7a94e7fc71f5a95938850ecc905323bb9d1
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>