cpufreq: ARM_BRCMSTB_AVS_CPUFREQ cannot be used with ARM_SCMI_CPUFREQ

The brcmstb-avs-cpufreq driver is considered a legacy driver and since
2018, ARCH_BRCMSTB systems have been using scmi-cpufreq. As a matter of
fact, when SCMI is in use, brcmstb-avs-cpufreq is unusable since the
SCMI firmware takes over, this can result in various problems, including
external synchronous aborts.

Express those constraints such that the driver is not enabled by default
when SCMI CPU frequency scaling is in use.

Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>