PM / devfreq: Add devfreq_get_devfreq_by_phandle()

This patch adds the new devfreq_get_devfreq_by_phandle() OF helper function
which can find the instance of devfreq device by using phandle ("devfreq").

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
[m.reichl and linux.amoon: Tested it on exynos4412-odroidu3 board]
Tested-by: Markus Reichl <m.reichl@fivetechno.de>
Tested-by: Anand Moon <linux.amoon@gmail.com>
Acked-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>