Input: imx6ul_tsc - generalize the averaging property

Make the avarage-samples property a general touchscreen property
rather than imx6ul device specific.

Signed-off-by: Guy Shapiro <guy.shapiro@mobi-wize.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>