Input: ads7846 - be more compatible with the hwmon framework

 - Hook up to hwmon
     * show sensor attributes only if hwmon is present
     * ... and the board's reference voltage is known
     * otherwise be just a touchscreen
 - Report voltages per hwmon convention
     * measure in millivolts
     * voltages are named in[0-8]_input (ugh)
     * for 7846 chips, properly range-adjust vBATT/in1_input

Battery measurements help during recharge monitoring.  On OSK/Mistral,
the measured voltage agreed with a multimeter to several decimal places.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>