boards: arm: sam4s_xplained: Add usart entry on table
boards: arm: sam4s_xplained: Add usart entry on table

Add usart entry on 'Supported Features' table.

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

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