ARM: dts: imx6ul: phyboard-segin: Rename dts to PHYTEC name scheme

Use the same name scheme for the phyBOARD-Segin and the phyCORE-i.MX
6UL as is used for the PHYTEC phyBOARD-Mira and phyCORE-i.MX 6.

This is only a cosmetic change and there is no functional change
intended.

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