ARM: dts: imx6ul: segin: Add boot media to dts filename

There is now a PHYTEC phyCORE-i.MX 6UL with eMMC instead of NAND flash
available. The dts filename needs to reflect that to differentiate both.

Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>