ramips: mt7621: use higher SPI clock speed on Mikrotik rbm11g and rbm33g

Previously the dts were using a value determined by empirical testing,
because of a spi driver/clock bug. The bug was fixed quite some time
ago. 33 MHz is the default clock frequency used by RouterBOOT and thus
safe.

Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>

target/linux/ramips/dts/mt7621_mikrotik_routerboard-m11g.dts

@@ -83,8 +83,7 @@
 	flash@0 {
 		compatible = "jedec,spi-nor";
 		reg = <0>;
-		// XXX empiric value to obtain actual 10MHz SCK at the chip
-		spi-max-frequency = <3125000>;
+		spi-max-frequency = <33000000>;
 
 		partitions {
 			compatible = "fixed-partitions";

target/linux/ramips/dts/mt7621_mikrotik_routerboard-m33g.dts

@@ -93,7 +93,7 @@
 	flash@0 {
 		compatible = "jedec,spi-nor";
 		reg = <0>;
-		spi-max-frequency = <3125000>;
+		spi-max-frequency = <33000000>;
 
 		partitions {
 			compatible = "fixed-partitions";
@@ -139,8 +139,7 @@
 	flash@1 {
 		compatible = "jedec,spi-nor";
 		reg = <1>;
-		// XXX empiric value to obtain actual 10MHz SCK at the chip
-		spi-max-frequency = <3125000>;
+		spi-max-frequency = <33000000>;
 
 		partitions {
 			compatible = "fixed-partitions";