ARM: davinci: fix davinci_cpufreq_init() declaration

The davinci_cpufreq_init() declaration is only seen by its caller
but not the definition:

drivers/cpufreq/davinci-cpufreq.c:153:12: error: no previous prototype for 'davinci_cpufreq_init'

Move it into the platform_data header that is already used an
interface between the two places.

Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/r/20230516153109.514251-2-arnd@kernel.org


Signed-off-by: Arnd Bergmann <arnd@arndb.de>