cpufreq: armada-37xx: fix frequency calculation for opp

The frequency calculation was based on the current(max) frequency of the
CPU. However for low frequency, the value used was already the parent
frequency divided by a factor of 2.

Instead of using this frequency, this fix directly get the frequency from
the parent clock.

Fixes: 92ce45fb ("cpufreq: Add DVFS support for Armada 37xx")
Cc: <stable@vger.kernel.org>
Reported-by: Christian Neubert <christian.neubert.86@gmail.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>