Clk: notifier handler for dynamic voltage scaling
Dynamic voltage and frequency scaling (dvfs) is a common power saving
technique in many of today's modern processors.  This patch introduces a
common clk rate-change notifier handler which scales voltage
appropriately whenever clk_set_rate is called on an affected clock.

There are three prerequisites to using this feature:

1) the affected clocks must be using the common clk framework
2) voltage must be scaled using the regulator framework
3) clock frequency and regulator voltage values must be paired via the
OPP library

If a platform or device meets these requirements then using the notifier
handler is straightforward.  A struct device is used as the basis for
performing initial look-ups for clocks via clk_get and regulators via
regulator_get.  This means that notifiers are subscribed on a per-device
basis and multiple devices can have notifiers subscribed to the same
clock.  Put another way, the voltage chosen for a rail during a call to
clk_set_rate is a function of the device, not the clock.

Signed-off-by: Mike Turquette <mturquette@linaro.org>