Clean up clock phase drift code a bit.

Clean up clock phase drift code a bit.

src/cpu/base.cc:
    Move clock phase drift code to the base CPU so that any CPU model can use it.
src/cpu/base.hh:
    Added two functions to help get the next cycle the CPU should be scheduled.
src/cpu/simple/atomic.cc:
src/cpu/simple/timing.cc:
    Use the function now in BaseCPU.

--HG--
extra : convert_revision : 444494b66ffc85fc473c23f57683c5f9458ad80c