diag: Register for IPC Router initialization callback

Diag driver needs to wait for IPC Router module to be initialized
before creating sockets. This is achieved by registering a
notification callback that is triggered when IPC Router module
is initialized. Sockets are created after this notification.

Change-Id: I4e5b31ede4d54f3f990e922d4aef4be5e9eb1895
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>