tests: drivers: adc: adc_api: enable nocache memory

The nucleo_f746zg board doesn't handle RX/TX buffers in
non-cacheable memory when required. This leads to an error
in the test_adc_api with the log:
Supplied buffer is not in a non-cacheable region.

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

Author: djiatsaf-st

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

@@ -0,0 +1 @@
+CONFIG_NOCACHE_MEMORY=y