dm: test: Add tests for the generic PHY uclass

Those tests check:
- the ability for a phy-user to get a phy based on its name or its index
- the ability of a phy device (provider) to manage multiple ports
- the ability to perform operations on the phy (init,deinit,on,off)
- the behavior of the uclass when optional operations are not implemented

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/sandbox/dts/test.dts

@@ -59,6 +59,23 @@
 		ping-add = <3>;
 	};
 
+	phy_provider0: gen_phy@0 {
+		compatible = "sandbox,phy";
+		#phy-cells = <1>;
+	};
+
+	phy_provider1: gen_phy@1 {
+		compatible = "sandbox,phy";
+		#phy-cells = <0>;
+		broken;
+	};
+
+	gen_phy_user: gen_phy_user {
+		compatible = "simple-bus";
+		phys = <&phy_provider0 0>, <&phy_provider0 1>, <&phy_provider1>;
+		phy-names = "phy1", "phy2", "phy3";
+	};
+
 	some-bus {
 		#address-cells = <1>;
 		#size-cells = <0>;