The lower-level driver may want to provide its own buffers. If so,
there's no need to allocate new ones.  This already happens to work
just fine (since we check for size of 0 and use devm allocation), but
it's good to document it.

[dianders: Resolved conflicts; documented that no code changes needed
on mainline]
Signed-off-by: NBill Richardson <wfrichar@chromium.org>
Signed-off-by: NDoug Anderson <dianders@chromium.org>
Reviewed-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>