staging: fsl-mc: add DPAA2 overview readme

add README file providing an overview of the DPAA2 architecture
and how it is integrated in Linux
Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/fsl-mc/TODO

-* Add README file (with ASCII art) describing relationships between
-  DPAA2 objects and how combine them to make a NIC, an LS2 switch, etc.
-  Also, define all acronyms used.
-
 * Decide if multiple root fsl-mc buses will be supported per Linux instance,
   and if so add support for this.