1. Generic Timer signals a PPI to GIC that calls ISR for guest context switch.
2. Time unit is microseconds.
3. Modify generic_tiomer.c, generic_timer.h, scheduler.c, scheduler.h, timer.c, timer.h, Makefile
4. Add asm-arm_inline.h, generic_timer_regs.h
Merge pull request #8 from in1004kyu/proto2-request

Guest context switch uses Timer interrupt.