boards: arm: samr21: Add active low CS gpio flags
boards: arm: samr21: Add active low CS gpio flags

Update cs-gpios flag with active low value to instruct spi driver how
to handle cs signal.

Signed-off-by: Gerson Fernando Budke <0ad1c92223f8f375c46e39c6141096f3d30896b4@gmail.com>

galak/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr