ARM: imx_v6_v7_defconfig: Enable the ADC part of the STMPE MFD

The SoM Apalis/Colibri iMX6 use the ADC of a STMPE 811. Enable
its driver.
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/configs/imx_v6_v7_defconfig

@@ -402,6 +402,7 @@ CONFIG_IIO=y
 CONFIG_MMA8452=y
 CONFIG_IMX7D_ADC=y
 CONFIG_RN5T618_ADC=y
+CONFIG_STMPE_ADC=y
 CONFIG_VF610_ADC=y
 CONFIG_SENSORS_ISL29018=y
 CONFIG_MAG3110=y