dt-bindings: arm: idle-states: Move exit-latency-us explanation

Move exit-latency-us explanation to exit-latency-us section.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>