cpufreq: retire the Integrator cpufreq driver

After switching the core module clocks controlling the Integrator
clock frequencies to the common clock framework, defining the
operating points in the device tree, and activating the generic
DT-based CPUfreq driver, we can retire the old Integrator
cpufreq driver.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>