spi/of: Add OF notifier handler

Add OF notifier handler needed for creating/destroying spi devices
according to dynamic runtime changes in the DT live tree. This code is
enabled when CONFIG_OF_DYNAMIC is selected.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: <linux-spi@vger.kernel.org>