Input: stmpe-ts - enforce device tree only mode

The STMPE MFD is only used with device tree configured systems (and STMPE
MFD core depends on OF), so force the configuration to come from device
tree only.

Tested-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>