cpufreq: sun50i: Add explicit include for cpu.h

cpufreq: sun50i: Add explicit include for cpu.h

Removing the include of cpu.h from of_device.h causes an error:

drivers/cpufreq/sun50i-cpufreq-nvmem.c:42:19: error: implicit declaration of function ‘get_cpu_device’; did you mean ‘get_device’? [-Werror=implicit-function-declaration]

As of_device.h is not otherwise needed, it can be replaced with of.h
(also implicitly included).

Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20230329-dt-cpu-header-cleanups-v1-15-581e2605fe47@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>