cpufreq: blacklist Arm Vexpress platforms in cpufreq-dt-platdev

Add "arm,vexpress" to cpufreq-dt-platdev blacklist since the actual
scaling is handled by the firmware cpufreq drivers(scpi, scmi and
vexpress-spc).

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>