Input: add SYN_MAX and SYN_CNT constants

SYN_* events are special and not enabled via set_bit() for devices. Hence,
they haven't been really needed, yet. However, user-space can still make
great use of that for int->string debugging helpers or alike.

Also, I haven't seen any reason not to define these, so here they are.
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

include/uapi/linux/input.h

@@ -194,6 +194,8 @@ struct input_keymap_entry {
 #define SYN_CONFIG		1
 #define SYN_MT_REPORT		2
 #define SYN_DROPPED		3
+#define SYN_MAX			0xf
+#define SYN_CNT			(SYN_MAX+1)
 
 /*
  * Keys and buttons