mx28evk: Move README file inside board directory

Board specific READMEs should be located inside the respective board directory.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
diff --git a/doc/README.mx28evk b/board/freescale/mx28evk/README
similarity index 100%
rename from doc/README.mx28evk
rename to board/freescale/mx28evk/README