Input: sgi_btns - add support for SGI Indy volume buttons

Also rename sgio2_btns to sgi_btns since the driver is not only
for SGI O2 anymore.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>