Use STM32 ADC generic bindings instead of legacy bindings on
DHCOR Testbench board.

The STM32 ADC specific binding to declare channels has been deprecated,
hence adopt the generic IIO channels bindings, instead.
The STM32MP151 device tree now exposes internal channels using the
generic binding. This makes the change mandatory here to avoid a mixed
use of legacy and generic binding, which is not supported by the driver.
Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>