powercap: idle_inject: Use higher resolution for idle injection

The resolution of the idle injection is limited to 1ms. If there is
a need for an injection of 1.2 ms, it is not possible.

The idle injection API is not yet used, so it is safe to convert the
existing API to the new time unit instead of adding more functions.

Convert to microsecond in order to use a finer grain time unit when
injecting idle cycles.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>