Let's define the voltatile registers (those that can't be cached) and
enable caching.  The rk808 is accessed almost constantly with cpufreq
so this is really nice.

As measured by ftrace:
  before this change: cpu0_set_target() => ~2200us
  after this change:  cpu0_set_target() =>  ~500us
Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>