Input: ads7846 - add pen_down sysfs attribute

It's handy for userspace diagnostics to see the pen down status, to
see whether the touchscreen is "stuck" (shortcircuited).

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Signed-off-by: Juha Yrjola <juha.yrjola@nokia.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>