MIPS: JZ4740: Move arch_init_irq out of arch/mips/jz4740/irq.c

MIPS: JZ4740: Move arch_init_irq out of arch/mips/jz4740/irq.c

In preparation for moving the JZ4740 interrupt controller driver to
drivers/irqchip, move arch_init_irq into setup.c such that everything
remaining in irq.c is related to said JZ4740 interrupt controller.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
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/10136/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>