ARM: dts: imx6ul: segin: Move ECSPI interface to board include file

The ECSPI interface is available on the expansion connector of every
PHYTEC phyBOARD-Segin. Move its definition to the board include file
for better reuse.

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