Input: add driver for Zeitec ZET6223

This is a basic driver for the Zeitec ZET6223 I2C touchscreen
controllers. The driver does not support firmware loading, which is not
required for all tablets which contain this chip.

Signed-off-by: Jelle van der Waa <jelle@vdwaa.nl>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>