arm: dts: exynos: Extend board description

u-boot uses the same DTS for the all Odroid XU3-based boards, so list
them in the model description to let user know that those boards are
supported.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>