arm64: dts: imx8mm-phg: Disable flexspi

The imx8mm-tqma8mqml SoM used on the PHG board does not come with
the QSPI flash populated, so disable it to avoid the following
error message:

spi-nor spi3.0: unrecognized JEDEC id bytes: 00 00 00 00 00 00

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