arch: arm: add macro definition for vector key write permit value
arch: arm: add macro definition for vector key write permit value

This commit adds a macro definition for the Vector Key value of
AIRCR register that permits the accompanying writes. The macro
is then used instead of the hard-coded numerical value.

Signed-off-by: Ioannis Glaropoulos <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

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