ARM Generic Timer

1. 调度 Event

2. 根据counter value，触发中断

3. 支持Virtualization 扩展

Generic Timer提供的system counter:

(1) 最少56bit 宽

(2) Frequency : 1--50MHz

Supports one or more alternative operating mode in which it increments by larger amounts at a lower freqency

(typically for power-saving)

(3) Roll-over : 至少40年，才会发生

(4) 精度: ARM本身不规定，但建议24小时内，误差不超过10秒

(5) Start-up : 从0开始计数