boards: frdm_kw41z: set VREF to alternate source
boards: frdm_kw41z: set VREF to alternate source

Since Voltage Reference block is not supported, the ADC does not work
properly on FRDM-KW41Z. Set voltage reference to alternate source
in frdm_kw41z_defconfig.

Signed-off-by: Johann Fischer <b39b26e1812eb092eb217277792f3735321fa456@phytec.de>

zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,explora26/zephyr,explora26/zephyr,punitvara/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,finikorg/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,ldts/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,explora26/zephyr,punitvara/zephyr