Input: wm97xx: split out touchscreen registering

wm97xx-core does several things in it initialization :
 - touchscreen input device setup
 - battery device creation

As the wm97xx is actually a multi-function device handling an audio
codec, a touchscreen, a gpio block and an ADC, reshape the probing to
isolate what is truly input/touchscreen specific from the remaining
part.

This is only code shuffling, there is no functional change.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>