mfd: stmpe: Move ADC related defines to MFD header

Move defines that are ADC related to the header of the overlying MFD,
so they can be used from multiple sub-devices.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>