README: mxs: Introduce README.mxs

Create a README.mxs file that contains instructions on how to use U-boot for
both MX23 and MX28.

As boot from NAND has only been tested on mx28, make it clear that it only
applies to MX28.

While at it, do some small cleanups for the sake of consistency:
- Use "MX28" instead of "i.MX28"
- Use "section" instead of "chapter" when referring to specific parts of the
reference manual chapters.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
diff --git a/doc/README.m28 b/doc/README.m28
index 2a92226..cb3ae20 100644
--- a/doc/README.m28
+++ b/doc/README.m28
@@ -9,5 +9,5 @@
 board/denx/m28evk/		- M28EVK board specific files
 include/configs/m28evk.h	- M28EVK configuration file
 
-Follow the instructions from doc/README.mx28_common to generate a bootable SD
-card or to boot from NAND flash.
+Follow the instructions from doc/README.mxs to generate a bootable SD card or to
+boot from NAND flash.