dt-bindings: SLIMBus: add slim devices optional properties

dt-bindings: SLIMBus: add slim devices optional properties

This patch adds an optional SLIMBus Interface device phandle property
that could be used by some of the SLIMBus devices.

Interface device is mostly used with devices that are dealing
with streaming.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200109103148.5612-2-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>