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/README

@@ -188,7 +188,7 @@ Load:   dtoverlay=at86rf233,<param>=<val>
 Params: interrupt               GPIO used for INT (default 23)
         reset                   GPIO used for Reset (default 24)
         sleep                   GPIO used for Sleep (default 25)
-        speed                   SPI bus speed in Hz (default 6000000)
+        speed                   SPI bus speed in Hz (default 3000000)
         trim                    Fine tuning of the internal capacitance
                                 arrays (0=+0pF, 15=+4.5pF, default 15)
 

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>;
 			};
 		};