slimbus: core: add of_slim_device_get() helper

On SLIMBus controllers like Qcom NGD(non ported device), controller
can request logical address once the remote side is powered, having a
helper function like this to explicitly enumerate the bus is helpful.
Also codec drivers which are taking to interface device would need
such a helper too.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>