dm: Add a dsi host uclass

Display Serial Interface (DSI) host can usefully be modelled
as their own uclass.
DSI defines a serial bus and a communication protocol
between the host and the device (panel, bridge).

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>

arch/sandbox/dts/test.dts

@@ -76,6 +76,10 @@
 		};
 	};
 
+	dsi_host: dsi_host {
+		compatible = "sandbox,dsi-host";
+	};
+
 	a-test {
 		reg = <0 1>;
 		compatible = "denx,u-boot-fdt-test";