tests: drivers: adc: Support RA8, RA6, RA4 to test accuracy and api

Add overlay to support test adc_accuracy_test and adc_api in boards:
EK-RA8D1, MCK-RA8T1, EK-RA6M1, EK-RA6M2, EK-RA6M3, EK-RA6M4, EK-RA6M5,
EK-RA6E2, FPB-RA6E1, FPB-RA6E2, EK-RA4M2, EK-RA4M3, EK-RA4E2, EK-RA4W1

Signed-off-by: Khoa Nguyen <khoa.nguyen.xh@renesas.com>