devfreq: devfreq_simple_dev: Add support for preparing device clock

For certain implementation, device clock needs to be prepared before
rate voting taking effect. Add support for preparing device clock
during initialization.

Change-Id: Ib22e83952187118342ff2546d4c79d3970a288f9
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>
[avajid@codeaurora.org: made minor styling changes]
Signed-off-by: Amir Vajid <avajid@codeaurora.org>