ads7846: Implement debouncing and rudimentary sample filtering

Some touchscreens seem to oscillate heavily for a while after
touching the screen.  Implement support for sampling the screen
until we get two consecutive values that are close enough.

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Signed-off-by: Juha Yrjölä <juha.yrjola@nokia.com>