Input: serio - support multiple child devices per single parent

Some (rare) serio devices need to have multiple serio children. One of
the examples is PS/2 multiplexer present on several TQC STKxxx boards,
which connect PS/2 keyboard and mouse to single tty port.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
diff --git a/include/linux/serio.h b/include/linux/serio.h
index 111ad50..109b237 100644
--- a/include/linux/serio.h
+++ b/include/linux/serio.h
@@ -41,7 +41,9 @@
 	int (*start)(struct serio *);
 	void (*stop)(struct serio *);
 
-	struct serio *parent, *child;
+	struct serio *parent;
+	struct list_head child_node;	/* Entry in parent->children list */
+	struct list_head children;
 	unsigned int depth;		/* level of nesting in serio hierarchy */
 
 	struct serio_driver *drv;	/* accessed from interrupt, must be protected by serio->lock and serio->sem */