mfd: Kconfig: MFD_SUN4I_GPADC depends on !TOUCHSCREN_SUN4I_GPADC

MFD_SUN4I_GPADC and TOUCHSCREEN_SUN4I are incompatible (both are drivers
for Allwinner SoCs' ADC). This makes sure TOUCHSCREEN_SUN4I isn't
enabled while MFD_SUN4I_GPADC is enabled.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>