scripts/dts: Simplify find_parent_prop()
scripts/dts: Simplify find_parent_prop()

Also replace a .get() where the key is known to exist.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

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