ramips: increase max SPI frequency to 50 MHz for EX3700/EX6130

Based on Macronix MX25L3205DM2I datasheet this is maximum
supported speed.

Signed-off-by: Frederik Noe-Sdun <[email protected]>
[rebased, improved commit title/message]
Signed-off-by: Adrian Schmutzler <[email protected]>

target/linux/ramips/dts/mt7620a_netgear_ex3700_ex6130.dtsi

@@ -39,7 +39,7 @@
 	m25p80@0 {
 		compatible = "jedec,spi-nor";
 		reg = <0>;
-		spi-max-frequency = <10000000>;
+		spi-max-frequency = <50000000>;
 
 		partitions {
 			compatible = "fixed-partitions";