PM / devfreq: exynos5: Add CONFIG_PM_OPP dependency to fix probe fail

This patch add CONFIG_PM_OPP dependecy to exynos5_bus driver
to fix probe fail. If CONFIG_PM_OPP is disabled, dev_pm_opp_find_freq_floor()
will always return ERR_PTR(-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>