of/platform: Make of_platform_device_destroy globally visible

of_platform_device_destroy is the counterpart to
of_platform_device_create which is a non-static function.

After creating a platform device it might be neccessary
to destroy it to deal with -EPROBE_DEFER where a
repeated of_platform_device_create call would fail otherwise.

Therefore also make of_platform_device_destroy globally visible.

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>