Add initial support for the AT42QT1050 (QT1050) device. The device
supports up to five input keys, dependent on the mode. Since it adds only
the initial support, the "1 to 4 keys plus Guard Channel" mode isn't
supported.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>