spmi: document spmi_device_from_of() refcounting

Add a comment documenting that the spmi_device_from_of() takes a
reference to the embedded struct device that needs to be dropped after
use.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20231003152927.15000-5-johan+linaro@kernel.org


Signed-off-by: Lee Jones <lee@kernel.org>