PM / devfreq: exynos: Update documentation for bus devices using passive governor

This patch updates the documentation for passive bus devices and adds the
detailed example of Exynos3250.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Acked-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>