slimbus: Callback to indicate device report present message

Clients can register a callback (just like probe) and that callback
will be called when the client's device reports present per slimbus
specification. The callback is hosted in a workqueue to avoid doing
it in same context from which probe is called (and/or in the context
receiving report present message). Callback is called when the device
reports present for the first time after the bus is initialized.

Change-Id: I8396b87121950f99a09764d497bf2d4b1535565d
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>