MIPS: Move FIXADDR_TOP into spaces.h

Memory maps and addressing quirks are normally defined in <spaces.h>.
There are already three targets that need to override FIXADDR_TOP, and
others exist.  This will be a cleaner approach than adding lots of
ifdefs in fixmap.h .

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/1573/
Acked-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>