ARM: dts: imx6qdl-tqma6: remove obsolete fsl,spi-num-chipselects

This property is unneeded and not supported by the spi-imx driver.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>