test: dm: add protocol-specific channel test

Any SCMI protocol may have its own channel.
Test this feature on sandbox as the necessary framework was added
in a prior commit.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 6abce9e..50c8715 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -739,6 +739,7 @@
 			clk_scmi: protocol@14 {
 				reg = <0x14>;
 				#clock-cells = <1>;
+				linaro,sandbox-channel-id = <0x14>;
 			};
 
 			reset_scmi: protocol@16 {