MIPS: Add missing includes to gic.h

The gic.h header uses bitmaps and NR_CPUS, and should therefore include
linux/bitmap.h and linux/threads.h. This is in preparation for use of
this header in a subsequent commit from a C file which doesn't already
include those headers.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6357/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>