MIPS: Loongson: Remove set_irq_trigger_mode()

set_irq_trigger_mode() is not needed on all platforms so remove it
and move the related source code to mach_init_irq().

This will allow gdium to share the common irq.c without adding an empty
set_irq_trigger_mode().

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1493/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>