ARM: dts: imx7d-flex-concentrator: Remove invalid ecspi property

The 'num-chipselects' property is not a valid property for ecspi.

Remove it to fix the following DT check warning:

spi@30630000: Unevaluated properties are not allowed ('num-chipselects' was unexpected)
From schema: Documentation/devicetree/bindings/spi/fsl-imx-cspi.yaml

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>