dt-bindings: devfreq: rk3399_dmc: Specify idle params in nanoseconds

It's inefficient to use the same number of cycles for all OPPs, since
lower frequencies make for longer idle times. Let's specify the idle
time instead, so software can pick the optimal number of cycles on its
own.

NB: these bindings aren't used anywhere yet.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>