cpufreq: armada-37xx: driver relies on cpufreq-dt

Armada-37xx driver registers a cpufreq-dt driver. Not having
CONFIG_CPUFREQ_DT selected leads to a silent abort during the probe.
Prevent that situation by having the former depending on the latter.

Fixes: 92ce45fb

 (cpufreq: Add DVFS support for Armada 37xx)
Cc: 4.16+ <stable@vger.kernel.org> # 4.16+
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>