devfreq: Support opp table for devfreq_simple_dev

Support read opp value for dev_target.
Use previous_freq instead of get_cur_freq when opp
table parsed success.

Change-Id: I37bb74ef18a822b05ed3be411bc59ae4dadb7a0d
Signed-off-by: Biao Long <blong@codeaurora.org>