dt-bindings: iio/adc: Add touchscreen idx for JZ47xx SoC ADC

Introduce support for touchscreen channels found in JZ47xx SoCs.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Tested-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>