lantiq: Slow down SPI flash on the DGN3500

The bootloader uses 30 MHz as the SPI frequency for flash on the Germany and
North America models, and 50 MHz for it on the worldwide model, but the Lantiq
SPI driver in OpenWrt and LEDE may access the flash differently such that
writes are capped at 20 MHz, leading to read errors reported on the worldwide
model at 30 MHz.

Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>
Acked-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>

target/linux/lantiq/dts/DGN3500.dtsi

@@ -168,7 +168,7 @@
 		#size-cells = <1>;
 		compatible = "jedec,spi-nor";
 		reg = <4 0>;
-		spi-max-frequency = <30000000>;
+		spi-max-frequency = <20000000>;
 
 		partitions {
 			compatible = "fixed-partitions";