Input: touchscreen: use wrapper for pxa2xx ac97 registers

To avoid a dependency on the pxa platform header files with
hardcoded registers, change the driver to call a wrapper
in the pxa2xx-ac97-lib that encapsulates all the other
ac97 stuff.

Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: linux-input@vger.kernel.org
Cc: alsa-devel@alsa-project.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>