BCM270X_DT: at86rf233 overlay - drop to 3MHz

The consensus is that 6MHz is too fast, but that 3MHz is OK.

See: #1294
     #1151

Author: Phil Elwell

arch/arm/boot/dts/overlays/at86rf233-overlay.dts

@@ -25,7 +25,7 @@
 				interrupts = <23 4>; /* active high */
 				reset-gpio = <&gpio 24 1>;
 				sleep-gpio = <&gpio 25 1>;
-				spi-max-frequency = <6000000>;
+				spi-max-frequency = <3000000>;
 				xtal-trim = /bits/ 8 <0xf>;
 			};
 		};