net: ipc_router: Add notifier support for AF_MSM_IPC registration

IPC Router registers AF_MSM_IPC address family during its module
initialization.

Add support to enable kernel drivers to register and to be notified about
the AF_MSM_IPC address family registration. This will help in ordering the
initialization sequence of IPC Router and its clients.

Change-Id: Id790f4eb6c1d814197fab12b7372860c365e6cb7
Signed-off-by: Atish Kumar Patra <apatra@codeaurora.org>