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.
I added the process for Cortex-A in mbed_application.c because this process was for only Cortex-M.
Also I enabled the macro of MBED_APPLICATION_SUPPORT for Cortex-A.
With "mov r2, #0", compile OK with GCC_ARM, but failed with ARMC6.
With "ldr r2, =0", compile OK with ARMC6, but failed with GCC_ARM.
Finally, with "movw r2, #0"/"movt r2, #0", compile OK with both ARMC6 and GCC_ARM.
Fix a calculation error in in powerdown_nvic preventing the last
group of 32 interrupts from getting powered down. The ARMv7m reference
manual states "the total number of interrupt lines is up
to (32*(INTLINESNUM+1)).".
Also rename isr_count to isr_groups_32 for clarity, since this is
actually the number of 32 interrupt groups.