Input: introduce BTN/ABS bits for drums and guitars

There are a bunch of guitar and drums devices out there that all report
similar data. To avoid reporting this as BTN_MISC or ABS_MISC, we
allocate some proper namespace for them. Note that most of these devices
are toys and we cannot report any sophisticated physics via this API.

I did some google-images research and tried to provide definitions that
work with all common devices. That's why I went with 4 toms, 4 cymbals,
one bass, one hi-hat. I haven't seen other drums and I doubt that we need
any additions to that. Anyway, the naming-scheme is intentionally done in
an extensible way.

For guitars, we support 5 frets (normally aligned vertically, compared to
the real horizontal layouts), a single strum-bar with up/down directions,
an optional fret-board and a whammy-bar.

Most of the devices provide pressure values so I went with ABS_* bits. If
we ever support devices which only provide digital input, we have to
decide whether to emulate pressure data or add additional BTN_* bits.

If someone is not familiar with these devices, here are two pictures which
provide almost all introduced interfaces (or try the given keywords
with a google-image search):
  Guitar: ("guitar hero world tour guitar")
    http://images1.wikia.nocookie.net/__cb20120911023442/applezone/es/images/f/f9/Wii_Guitar.jpg
  Drums: ("guitar hero drums")
    http://oyster.ignimgs.com/franchises/images/03/55/35526_band-hero-drum-set-hands-on-20090929040735768.jpg



Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>