[MIPS] Fix FIXADDR_TOP for TX39/TX49.

FIXADDR_TOP is used for HIGHMEM and for upper limit of vmalloc area on
32bit kernel.  TX39XX and TX49XX have "reserved" segment in CKSEG3
area.  0xff000000-0xff3fffff on TX49XX and 0xff000000-0xfffeffff on
TX39XX are reserved (unmapped, uncached) therefore can not be used as
mapped area.

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