ARM: dts: imx6q-cm-fx6: Remove invalid SPI flash entry

"st,m25p" is not a valid compatible according to jedec,spi-nor.yaml.

Remove it.

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