cpufreq: brcmstb-avs-cpufreq: send S2_ENTER / S2_EXIT commands to AVS

On suspend we send AVS_CMD_S2_ENTER and on resume AVS_CMD_S2_EXIT.
These are best effort calls, so we don't check the return code or take
any action if either of the calls fails.

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>