ARM: dts: stm32: add qspi flash compatible string for stm32f769-disco

Add missing flash compatible string to be able to read/write into
qspi flash.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>

arch/arm/dts/stm32f769-disco-u-boot.dtsi

@@ -159,6 +159,7 @@
 	flash0: mx66l51235l {
 		#address-cells = <1>;
 		#size-cells = <1>;
+		compatible = "jedec,spi-nor";
 		spi-max-frequency = <108000000>;
 		spi-rx-bus-width = <4>;
 		reg = <0>;