MIPS: Move GIC to drivers/irqchip/

Move GIC irqchip support to drivers/irqchip/ and rename the Kconfig
option from IRQ_GIC to MIPS_GIC to avoid confusion with the ARM GIC.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Reviewed-by: Qais Yousef <qais.yousef@imgtec.com>
Tested-by: Qais Yousef <qais.yousef@imgtec.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jeffrey Deans <jeffrey.deans@imgtec.com>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Jonas Gorski <jogo@openwrt.org>
Cc: John Crispin <blogic@openwrt.org>
Cc: David Daney <ddaney.cavm@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/7812/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>