amd64: add delay_fini()

amd64: add delay_fini()

Not all of the clocks with a delay(9) implementation necessarily keep
ticking across suspend/resume.  We need a clean way to reverse
delay_init() during suspend when those clocks stop ticking.

Hence, delay_fini().  delay_fini() resets delay_func() to
i8254_delay() if the given function pointer is the active delay(9)
implementation.

ok mlarkin@