[MIPS] Fix some sparse warnings on traps.c and irq-msc01.c

* Declare board_bind_eic_interrupt, board_watchpoint_handler in traps.h
* Make msc_bind_eic_interrupt static and fix its argument types.
* Make msc_levelirq_type, msc_edgeirq_type static.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>