timer: Add a test for timer_timebase_fallback

timer: Add a test for timer_timebase_fallback

To test this function, sandbox CPU must set cpu_platdata.timebase_freq on
bind. It also needs to expose a method to set the current cpu. I also make
some most members of cpu_sandbox_ops static.

On the timer side, the device tree property
sandbox,timebase-frequency-fallback controls whether sandbox_timer_probe
falls back to time_timebase_fallback or to SANDBOX_TIMER_RATE.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>