iio: adc: add STMPE ADC devicetree bindings

This adds the devicetree bindings for the STMPE ADC. This also corrects
a typo in st,sample-time it is rather "6 -> 124 clocks" according
to the datasheet and not 144.
We need to use the naming stmpe_adc in devicetree because this is given
by the mfd device.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>