cpufreq: brcmstb-avs-cpufreq: more flexible interface for __issue_avs_command()

We are changing how parameters are passed to __issue_avs_command(), so we
can pass input *and* output arguments with the same command, rather than
just one or the other.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>