treewide: Remove OF_PRIOR_STAGE

The previous patches removed OF_PRIOR_STAGE from the last consumers of the
Kconfig option.  Cleanup any references to it in documentation,  code and
configuration options.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>