MIPS: include linux/types.h

The file uses u16 type but doesn't include its definition explicitly

I was getting this error when including this header in my driver:

  arch/mips/include/asm/mipsregs.h:644:33: error: unknown type name ‘u16’

Signed-off-by: Qais Yousef <qais.yousef@imgtec.com>
Reviewed-by: Steven J. Hill <Steven.Hill@imgtec.com>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6212/