mx28evk: README: Fix location for the generic mxs README

doc/README.mxs no longer exists. It has been renamed doc/imx/common/mxs.txt,
so fix the mx28evk README accordingly.
Signed-off-by: NFabio Estevam <festevam@gmail.com>

board/freescale/mx28evk/README

@@ -58,5 +58,5 @@ It is possible to solder a SOIC memory on U49 or use a DIP8 on J89.
 To get SPI communication to work R320, R321,R322 and C178 need to be populated.
 Look in the schematics for the proper component values.
 
-Follow the instructions from doc/README.mxs to generate a bootable SD card or
-to generate a binary to be flashed into SPI NOR.
+Follow the instructions from doc/imx/common/mxs.txt to generate a bootable
+SD card or to generate a binary to be flashed into SPI NOR.