arm64: dts: imx8mn-beacon-som: Enable QSPI on SOM

There is a QSPI chip connected to the FlexSPI bus.  Enable it.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>