Kernel: Add a method to determine the desired permissions of a Device

This method will be used later in DevFS, to set the appropriate
permissions for each device node.

Kernel/Devices/RandomDevice.h

@@ -36,6 +36,9 @@ public:
     RandomDevice();
     virtual ~RandomDevice() override;
 
+    // ^Device
+    virtual mode_t required_mode() const override { return 0666; }
+
 private:
     // ^CharacterDevice
     virtual KResultOr<size_t> read(FileDescription&, size_t, UserOrKernelBuffer&, size_t) override;