boards: atsamd20_xpro: add cs-gpio for one spi device
boards: atsamd20_xpro: add cs-gpio for one spi device

The samd20 does not have hardware support for controlling the SS pin.
Declare a cs-gpio to be able to use the SS pin for samd20 xplained pro
board.

Signed-off-by: Sean Nyekjaer <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@geanix.com>

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