cmake: dtc: silence output from check_dtc_flag
cmake: dtc: silence output from check_dtc_flag

We'd get output from a cmake build that would have lines like:

Version: DTC 1.4.7
Version: DTC 1.4.7

This was a side effect of check_dtc_flag.  Add the OUTPUT_QUIET option
to execute_process to supress this output.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

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