hw/adc: Add an ADC module for NPCM7XX

The ADC is part of NPCM7XX Module. Its behavior is controled by the
ADC_CON register. It converts one of the eight analog inputs into a
digital input and stores it in the ADC_DATA register when enabled.

Users can alter input value by using qom-set QMP command.

Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>
Reviewed-by: Tyrone Ting <kfting@nuvoton.com>
Signed-off-by: Hao Wu <wuhaotsh@google.com>
Message-id: 20210108190945.949196-4-wuhaotsh@google.com
[PMM: Added missing hw/adc/trace.h file]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>