tests: drivers: adc: adc_api: increase adc prescaler

increase value of the adc-prescaler to avoid ADC
overrun.

adc driver related error log:
"E: ADC overrun error occurred ..."

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>

tests/drivers/adc/adc_api/boards/nucleo_f746zg.overlay

@@ -17,6 +17,7 @@
 	dma-names = "dma";
 
 	pinctrl-0 = <&adc1_in0_pa0 &adc1_in3_pa3>;
+	st,adc-prescaler = <4>;
 	#address-cells = <1>;
 	#size-cells = <0>;