The CN63XXP1 needs a couple of workarounds to ensure memory is not written
in unexpected ways.

All PREF with hints in the range 0-4,6-24 are replaced with PREF 28.  We
pass a flag to the assembler to cover compiler generated code, and patch
uasm for the dynamically generated code.

The write buffer threshold is reduced to 4.
Signed-off-by: NDavid Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/1672/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>