cpufreq: vexpress: Stop setting cpufreq_driver->attr field

The cpufreq core now handles this for basic attributes, including boost
frequencies, the driver can skip setting them.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>