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>