slimbus: Add slave devices to controller after controller HW is up

Register controller is only responsible for making sure slimbus
controller structure is initialized with framework. This should not
assume that slimbus controller's hardware programming is done (e.g. if
slave's probe is trying to get logical address, then that will fail if
controller is registered, but slimbus hardware programming is not
complete, since probe is called synchrounously when device is added.
So this API separates register controller from slave device
registration. It should be called by controller when its hardware
programming is complete.

Change-Id: I52fa5567fc072951c89f99c70507aa3b1e4e22af
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>