Input: ads7846 - document that it handles tsc2046 too

The TSC2046 is an updated version of the ADS7846 ... mention that in
the Kconfig helptext and driver source.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Kevin Hilman <khilman@mvista.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>