irqchip: mips-gic: Use proper iomem accessors

Get rid of the ugly GICREAD/GICWRITE/GICBIS macros and use proper
iomem accessors instead.  Since the GIC registers are not directly
accessed outside of the GIC driver any more, make gic_base static
and move all the GIC register manipulation macros out of gic.h,
converting them to static inline functions.

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/8127/
Patchwork: https://patchwork.linux-mips.org/patch/8229/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>