MIPS: Netlogic: Fixup memory regions for prefetch

Fix a cache error found in stress test, caused by the prefetch instruction
going beyond valid memory when acessing the last page of a region. Add
the pref_backup logic similar to XLR in XLP too.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5431/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>