Input: Add and document BTN_GRIP*

Input: Add and document BTN_GRIP*

Many controllers these days have started including grip buttons. As
there has been no particular assigned BTN_* constants for these, they've
been haphazardly assigned to BTN_TRIGGER_HAPPY*. Unfortunately, the
assignment of these has varied significantly between drivers.

Add and document new constants for these grip buttons.

Signed-off-by: Vicki Pfau <vi@endrift.com>
Link: https://lore.kernel.org/r/20250702040102.125432-2-vi@endrift.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>