Input: xpad - allow using triggers as buttons rather than axes

Certain devices implement triggers as buttons rather than axes. In
particular, arcade sticks such as the HORI Real Arcade Pro.EX do not
have analog buttons.

These devices are now setup to present buttons rather than axes for
triggers.

User-space applications often also have problems with
axes-as-buttons. Activating MAP_TRIGGERS_TO_BUTTONS for a device
removes the artificial difference between buttons and triggers.

Signed-off-by: Nicolas Léveillé <nicolas@uucidl.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>