dm: Add support for simple-pm-bus

This type of bus is used in Linux to designate buses which have power
domains and/or clocks which need to be enabled before their child devices
can be used. Because power domains are automatically enabled before probing
in U-Boot, we just need to enable any clocks present.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index a6e2bfd..b59bf15 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -1038,6 +1038,12 @@
 	mdio: mdio-test {
 		compatible = "sandbox,mdio";
 	};
+
+	pm-bus-test {
+		compatible = "simple-pm-bus";
+		clocks = <&clk_sandbox 4>;
+		power-domains = <&pwrdom 1>;
+	};
 };
 
 #include "sandbox_pmic.dtsi"