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 <human.hwang@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>