hw/timer/cmsdk-apb-dualtimer: Implement CMSDK dual timer module

hw/timer/cmsdk-apb-dualtimer: Implement CMSDK dual timer module

The Arm Cortex-M System Design Kit includes a "dual-input timer module"
which combines two programmable down-counters. Implement a model
of this device.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180820141116.9118-4-peter.maydell@linaro.org