cmake: dts: Fix building with dtc 1.4.6
cmake: dts: Fix building with dtc 1.4.6

dtc 1.4.6 doesn't support the -Wno-unique_unit_address flag.  We need to
check that the flag is supported before using it on pre dtc-1.4.7
versions.

Fixes #12685

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

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