arm: dts: mediatek: add quad mode capabilities for SPI flashes

Explicitly add quad mode capabilities or the SPI controller may
start transfer in single mode.

diff --git a/arch/arm/dts/mt7986b-rfb.dts b/arch/arm/dts/mt7986b-rfb.dts
index 8196845..f98b04a 100644
--- a/arch/arm/dts/mt7986b-rfb.dts
+++ b/arch/arm/dts/mt7986b-rfb.dts
@@ -177,12 +177,16 @@
 		compatible = "jedec,spi-nor";
 		reg = <0>;
 		spi-max-frequency = <52000000>;
+		spi-rx-bus-width = <4>;
+		spi-tx-bus-width = <4>;
 	};
 
 	spi_nand@1 {
 		compatible = "spi-nand";
 		reg = <1>;
 		spi-max-frequency = <52000000>;
+		spi-rx-bus-width = <4>;
+		spi-tx-bus-width = <4>;
 	};
 };