iio: exynos-adc: add experimental touchscreen support

This adds support for the touchscreen on Samsung s3c64xx.
The driver is completely untested but shows roughly how
it could be done, following the example of the at91 driver.

compared to the old plat-samsung/adc driver, there is
no support for prioritizing ts over other clients, nor
for oversampling. From my reading of the code, the
priorities didn't actually have any effect at all, but
the oversampling might be needed.

Verifying this driver is the main issue that is currently
holding up multiplatform support for s3c64xx, so any help
in testing is very much appreciated.

The current version uses the IS_REACHABLE() that is
going to be introduced in the linux-media tree, please
comment this out for testing.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>