Disable usage of PREF instruction usage by memcpy for MIPS R6.

MIPS R6 redefines PREF instruction with smaller offset than
ordinary MIPS. However, the memcpy code uses PREF instruction
with offsets bigger than +-256 bytes.

Malta kernels already disable usage of PREF for memcpy.

This was found during adaptation of MIPS R6 for virtual board
used by Android emulator.
Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtec.com>
Signed-off-by: Goran Ferenc <goran.ferenc@imgtec.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtech.com>
Cc: James.Hogan@imgtec.com
Cc: Paul.Burton@imgtec.com
Cc: Raghu.Gandham@imgtec.com
Cc: Leonid.Yegoshin@imgtec.com
Cc: Douglas.Leung@imgtec.com
Cc: Petar.Jovanovic@imgtec.com
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16510/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>