docs/system: Remove ADC from raspi documentation

None of the RPi boards have ADC on-board. In real life, an external ADC chip
is required to operate on analog signals.

Signed-off-by: Rayhan Faizel <rayhan.faizel@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240512085716.222326-1-rayhan.faizel@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
1 file changed