arch/aarch64: Return error if IAR queried outside ISR

The AArch64 interrupt interface is currently inconsistent with the
Arm-m interface in that it returns FWK_SUCCESS if the current
interrupt ID is queried outside of an interrupt handler.

Add logic to arch_interrupt_get_current to return FWK_E_STATE instead in
this condition.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>