of: Move of_device_(add|register|unregister) to of_platform.h

As of_device_(add|register|unregister) functions work on struct
platform_device, they should be declared in of_platform.h instead.

This move is transparent for now as both headers include each other.

Link: https://lore.kernel.org/r/20230329-dt-cpu-header-cleanups-v1-2-581e2605fe47@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>