Odroid-XU3: Add documentation for Odroid-XU3

Odroid-XU3: Add documentation for Odroid-XU3

This patch adds documentation for Odroid-XU3. This documentation is
based on that of Odroid (doc/README-odroid) made by Przemyslaw Marczak.
The documentation includes basic information about boot media layout,
environment, partition layout, and the instruction to burn the u-boot
image to boot media.

Signed-off-by: Hyungwon Hwang <[email protected]>
Acked-by: Simon Glass <[email protected]>
Tested-by: Kevin Hilman <[email protected]>
Signed-off-by: Minkyu Kang <[email protected]>