boards: arm: atsamr21_xpro: Fix corrupted frames on rf2xx

When using internal IEEE 802.15.4 transceiver AT86RF233 at SPI speeds
above 6MHz was detected that frame buffer read returns corrupted data.
This set spi-max-frequency to 6MHz at atsamr21_xpro.dts to ensure best
relation of performance vs reliability.

Signed-off-by: Gerson Fernando Budke <[email protected]>

Author: nandojve

boards/arm/atsamr21_xpro/atsamr21_xpro.dts

@@ -81,7 +81,7 @@
 		compatible = "atmel,rf2xx";
 		reg = <0x0>;
 		label = "RF2XX_0";
-		spi-max-frequency = <8000000>;
+		spi-max-frequency = <6000000>;
 		irq-gpios = <&portb 0 (GPIO_ACTIVE_HIGH | GPIO_PULL_DOWN)>;
 		reset-gpios = <&portb 15 GPIO_ACTIVE_LOW>;
 		slptr-gpios = <&porta 20 GPIO_ACTIVE_HIGH>;