of: Move of_device_{add,register,unregister} to platform.c

The declarations for of_device_{add,register,unregister} were moved into
of_platform.h, so the implementations should be moved to platform.c as
well.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20230717143718.1715773-2-robh@kernel.org


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