hw/misc: Add QTest for NPCM7XX PWM Module

We add a qtest for the PWM in the previous patch. It proves it works as
expected.

Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>
Reviewed-by: Tyrone Ting <kfting@nuvoton.com>
Signed-off-by: Hao Wu <wuhaotsh@google.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20210108190945.949196-6-wuhaotsh@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>