mfd: stmpe: Preparations for STMPE ADC driver

This prepares the MFD for the STMPE ADC driver. This commit introduces
devicetree settings that are used by the ADC and adds an init function.
Common ADC settings that are shared with the touchscreen driver can now
reside in the overlying MFD.

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>
Signed-off-by: Lee Jones <lee.jones@linaro.org>