ARM: dts: imx6q-kontron-samx6i: Remove old fsl,spi-num-chipselects

The property "fsl,spi-num-chipselects" is gone since commit 790739c4
("dt-bindings: spi: Convert imx cspi to json-schema").

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>