doc: Fix CONFIG_OF_SEPARATE description

CONFIG_OF_SEPARATE description is not in align with actual code in Makefile and
thus has misleading instructions and explanation.

Make it aligned with the actual code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed