cpufreq: dt: support checking initial rate

cpufreq: dt: support checking initial rate

Bootloader or kernel sets CPU frequency to an initial value before cpufreq
starts on rockchip platform, if cpu's opp table is modified to a specified
value, it will cause an issue.

For example, the initial frequency is 816MHz and voltage set by hardware
is 900mV:
1. there is only one opp whose frequency is 816MHz and voltage is 850mV
in opp table list, as they frequency is equal, the voltage will not be
changed, it is still 900mV and a little too large relative to 850mV.
2. there is only one opp whose frequency is 1200MHz and voltage is 1100mV
in opp table list, as it doesn't set voltage to 1100mV before set frequency
to 1200MHz in the dev_pm_opp_set_rate function, the initial voltage 900mV
cann't supply for 1200MHz, the system crash.

Change-Id: Iba41536367ba5802dd8f7f37e245f0e5781eb643
Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>