firmware: smci: sandbox test for SCMI reset controllers

Add tests for SCMI reset controllers. A test device driver
sandbox-scmi_devices.c is used to get reset resources, allowing further
resets manipulation.

Change sandbox-smci_agent to emulate 1 reset controller exposed through
an agent. Add DM test scmi_resets to test this reset controller.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 5ed364f..e6da47d 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -376,6 +376,11 @@
 				reg = <0x14>;
 				#clock-cells = <1>;
 			};
+
+			reset_scmi0: protocol@16 {
+				reg = <0x16>;
+				#reset-cells = <1>;
+			};
 		};
 
 		sandbox-scmi-agent@1 {
@@ -1082,6 +1087,7 @@
 	sandbox_scmi {
 		compatible = "sandbox,scmi-devices";
 		clocks = <&clk_scmi0 7>, <&clk_scmi0 3>, <&clk_scmi1 1>;
+		resets = <&reset_scmi0 3>;
 	};
 
 	pinctrl {