MIPS: JZ4740: parse SoC interrupt controller parent IRQ from DT

MIPS: JZ4740: parse SoC interrupt controller parent IRQ from DT

Rather than hardcoding the IRQ number used to cascade interrupts from
the SoC interrupt controller to the CPU interrupt controller, read that
IRQ number from the DT describing the system.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: Brian Norris <computersforpeace@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/10137/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>