cpufreq: dbx500: Delete obsolete driver

We have moved the Ux500 over to use the generic DT based
cpufreq driver, so delete the old custom driver.

At the same time select CPUFREQ_DT from the machine's
Kconfig in order to satisfy the "default ARCH_U8500"
selection on the old 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>