ARM: dts: stm32: Set spi-rx/tx-bus-width to 4 for stm32f769-disco

As mx66l512 qspi flash supports quad input fast program and
quad input fast read, set spi-tx_bus-width and spi-rx_bus-width
to 4.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/arch/arm/dts/stm32f769-disco-u-boot.dtsi b/arch/arm/dts/stm32f769-disco-u-boot.dtsi
index 1e8cd6d..53a645d 100644
--- a/arch/arm/dts/stm32f769-disco-u-boot.dtsi
+++ b/arch/arm/dts/stm32f769-disco-u-boot.dtsi
@@ -161,6 +161,7 @@
 		#size-cells = <1>;
 		compatible = "jedec,spi-nor";
 		spi-max-frequency = <108000000>;
+		spi-tx-bus-width = <4>;
 		spi-rx-bus-width = <4>;
 		reg = <0>;
 	};