boards: arm: atsamd21: enable USB in dts
boards: arm: atsamd21: enable USB in dts

USB is already being configured in pinmux.c, but for it to be
availiable it also needs to be enabled in the device tree.

Signed-off-by: Benjamin Valentin <1876e1c5468f589edaf83253d033ba94c5f15c80@ml-pa.com>

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