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>