Improve the process for Cortex-A9 in mbed_application
Improve the process for Cortex-A9 in mbed_application

I added the powerdown func by GIC in mbed_application because Cortex-A9 use GIC instead of NVIC.
This process prevent unexpected interrupt when updating software by using bootloader.

betzw/mbed-os,betzw/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,betzw/mbed-os,betzw/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,betzw/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,betzw/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,kjbracey-arm/mbed