PM / devfreq: Add show_one macro to delete the duplicate code

This patch adds the 'show_one' macro to simplify the duplicate code
of both max_freq_show() and min_freq_show().

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>