sandbox: Make sandbox,emul more conventional

At present this property is a phandle but does not have a #xxx-cells
property to match it. Add one so that is works the same as gpio and clock
phandles.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/driver-model/pci-info.rst

@@ -125,6 +125,7 @@ emulator driver. For example::
 		compatible = "sandbox,pci-emul-parent";
 		emul_1f: emul@1f,0 {
 			compatible = "sandbox,swap-case";
+			#emul-cells = <0>;
 		};
 	};