ramips: increase spi-max-frequency for ipTIME mt7620 devices

This commit increases the hardware SPI frequency from 24.2MHz to 48.3MHz.

[    5.314163] m25p80 spi0.0: speed: 24166666/40000000, rate: 8, prescal: 2, loops: 226
[    5.076323] m25p80 spi0.0: speed: 48333333/50000000, rate: 4, prescal: 1, loops: 162

`time cat /dev/mtd2 >/dev/null` is reduced from 5.64s to 4.36s on A104ns,
and from 11.39s to 8.81s on A1004ns.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>

target/linux/ramips/dts/mt7620a_iptime.dtsi

@@ -21,7 +21,7 @@
 	flash@0 {
 		compatible = "jedec,spi-nor";
 		reg = <0>;
-		spi-max-frequency = <40000000>;
+		spi-max-frequency = <50000000>;
 
 		partitions {
 			compatible = "fixed-partitions";