irqchip/sun4i: Fix compilation outside of arch/arm

The Allwinner sunxi specific interrupt controller cannot be compiled
for any architecture except arm:

drivers/irqchip/irq-sun4i.c:25:26: fatal error: asm/mach/irq.h: No such file or directory
compilation terminated.

It turns out that this header is actually not needed for the driver, so remove
it and allow compilation for other architectures like arm64.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: linux-sunxi@googlegroups.com
Link: http://lkml.kernel.org/r/1454348370-3816-2-git-send-email-andre.przywara@arm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>