mfd: rk808: Add register caching

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>