ARM: dts: imx6qdl-kontron-samx6i: fix SPI0 chip selects

There is a comment in the imx6q variant dtsi claiming that these
modules will have one more chip select than the imx6dl variant.
This is wrong. Ordinary GPIOs are used for chip selects and both
variants of the module share the very same PCB and both have this
GPIO routed to the SPI0_CS1# pin of the SMARC connector.

Fix it by moving the third chip select description to the common dtsi.

Fixes: 21252127 ("ARM: dts: imx6qdl-kontron-samx6i: add Kontron SMARC SoM Support")
Signed-off-by: Michael Walle <mwalle@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>