Add the STM32F2xx ADC device. This device randomly
generates values on each read.

This also includes creating a hw/adc directory.

Signed-off-by: Alistair Francis <alistair@alistair23.me>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 3240e660adaf537f55a63ce06096e844aece8cda.1474742262.git.alistair@alistair23.me
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>