Add minimal ADC driver code for EK-RA8M1 board

Signed-off-by: Duy Phuong Hoang. Nguyen <duy.nguyen.xa@renesas.com>
Signed-off-by: Tri Nguyen <tri.nguyen.wj@bp.renesas.com>
Signed-off-by: Thao Luong <thao.luong.uw@renesas.com>