ref-manual: Edits to "atom-pc Board Support Package (BSP)" section.

Fixes [YOCTO #6400]

Replaced the sentence in the note to specifically note that the
genericx86-64 BSP has been added for 64-bit Atom systems.

(From yocto-docs rev: 8b238e3aa2019803cc30a00042bae1a2464bd792)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/migration.xml

@@ -627,8 +627,8 @@
             hardware, but it will run on a wider range of systems than the
             <filename>atom-pc</filename> did.
             <note>
-                Additionally, a <filename>genericx86-64</filename> BSP has been
-                added for 64-bit systems.
+                Additionally, a <filename>genericx86-64</filename> BSP has
+                been added for 64-bit Atom systems.
             </note>
         </para>
     </section>