doc: board: samsung: exynos-mobile: remove requirement of stub device tree

doc: board: samsung: exynos-mobile: remove requirement of stub device tree

Flashing U-Boot for Exynos 7870 requires creating a stub device tree,
where certain properties and nodes are defined which are populated by
the previous bootloader in the phones.

Since these properties are now available in the U-Boot device tree, it's
now possible to use the same blob generated by U-Boot in place of the
stub, when creating boot images. Update the build documentation to
reflect the same.

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>