MIPS: Move gic.h to include/linux/irqchip/mips-gic.h

MIPS: Move gic.h to include/linux/irqchip/mips-gic.h

Now that the MIPS GIC irqchip lives in drivers/irqchip/, move
its header over to include/linux/irqchip/.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Qais Yousef <qais.yousef@imgtec.com>
Cc: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8129/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>