ARM: ux500: don't select CPUFREQ_DT

The new 'select CPUFREQ_DT' statement causes a warning in some
configurations:

warning: (ARCH_U8500) selects CPUFREQ_DT which has unmet direct dependencies (CPU_FREQ && HAVE_CLK && OF && (!CPU_THERMAL || THERMAL))

No other platform does this, so I think for consistency it's
best if we remove it again from ux500 and instead add the driver
to the defconfig.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>