arm64: use IRQ_STACK_SIZE instead of THREAD_SIZE for irq stack

IRQ_STACK_SIZE can be made different from THREAD_SIZE,
and as IRQ_STACK_SIZE is used while irq stack allocation,
same define should be used while printing information of irq stack.

Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/1596196190-14141-1-git-send-email-maninder1.s@samsung.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>