cpufreq: armada-37xx: Add missing MODULE_DEVICE_TABLE

CONFIG_ARM_ARMADA_37XX_CPUFREQ is tristate option and therefore this
cpufreq driver can be compiled as a module. This patch adds missing
MODULE_DEVICE_TABLE which generates correct modalias for automatic
loading of this cpufreq driver when is compiled as an external module.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: 92ce45fb

 ("cpufreq: Add DVFS support for Armada 37xx")
[ Viresh: Added __maybe_unused ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>