Input: wistron - introduce generic keymap

It turns out that the keymaps in the wistron driver are almost the
same, the main difference being some keys which may not exist and
leds which might not be present. Therefore it's possible to write
a generic keymap which would allow the use of an unknown keyboard
with little drawbacks. The user can select it specifying the parameter
"keymap=generic".

Signed-off-by: Eric Piel <eric.piel@tremplin-utc.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>