arm64: dts: imx8mm-phyboard-polis-rdk: Remove 'fsl,spi-num-chipselects'

The 'fsl,spi-num-chipselects' property is not a valid one, so remove it.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Yannic Moog <y.moog@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>