tile: implement dynamic frequency changing

tile: implement dynamic frequency changing

This change provides a cpufreq driver for tilegx, and also the
associated machinery to support the fact that we use the core frequency
as the actual timekeeper clock, so we have to go to some trouble to keep
everything lined up as we change the core frequency.

A newer hypervisor is required that implements the hv_set_speed() API.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>