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 <nandojve@gmail.com>