[U-Boot] [PATCH] README.mx28: add

[Marek Vasut]

Dear Otavio Salvador,

> This is mainline a copy of README.m28 changing the references to the
> mx28 counterpart but it makes easy for people looking for information
> about mx28 based boards.
> 
> Signed-off-by: Otavio Salvador <otavio at ossystems.com.br>
> Cc: Fabio Estevam <fabio.estevam at freescale.com>

This is copy-paste schizzle, can you please separate the CPU specific parts into 
one file and put only the different parts into their particular readmes?

Thanks!

Best regards,
Marek Vasut