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/TTY/TTY.h

@@ -67,6 +67,9 @@ public:
     void set_default_termios();
     void hang_up();
 
+    // ^Device
+    virtual mode_t required_mode() const override { return 0620; }
+
 protected:
     virtual ssize_t on_tty_write(const UserOrKernelBuffer&, ssize_t) = 0;
     void set_size(unsigned short columns, unsigned short rows);