[MIPS] i8295 cleanups.

 o Move i8259 function declarations to include/asm-mips/i8259.h
 o Make i8259.c functions static where possible.

Signed-off-by: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>