BCM270X_DT: Reduce default at86rf233 SPI frequency

The AT86RF233 has been found to be unreliable on a 7.5MHz SPI bus,
so reduce the default frequency to 6MHz.

See: #1151

Author: Phil Elwell

arch/arm/boot/dts/overlays/README

@@ -178,7 +178,7 @@ 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 750000)
+        speed                    SPI bus speed in Hz (default 6000000)
         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 = <7500000>;
+				spi-max-frequency = <6000000>;
 				xtal-trim = /bits/ 8 <0xf>;
 			};
 		};