cpufreq: brcmstb-avs-cpufreq: Support polling AVS firmware

In case the interrupt towards the host is never raised, yet the AVS
firmware responds correctly within the alloted time, allow supporting a
polling mode.

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