PM / devfreq: exynos4: Add CONFIG_PM_OPP dependency to fix probe fail

This patch add CONFIG_PM_OPP dependecy to exynos4_bus driver
to fix probe fail as following log:

[    3.721389] exynos4-busfreq busfreq.3: Fail to add opp entries.
[    3.721697] exynos4-busfreq: probe of busfreq.3 failed with error -22

If CONFIG_PM_OPP is disabled, dev_pm_opp_find_freq_floor() in xxx_probe()
will always return -EINVAL error.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>