mips: Add CP0 Write Merge config support

CP0 config register may indicate whether write-through merging
is allowed. Currently there are two types of the merging available:
SysAD Valid and Full modes. Whether each of them are supported by
the core is implementation dependent. Moreover whether the ability
to change the mode also depends on the chip family instance. Taking
into account all of this we created a dedicated mm_config() method
to detect and enable merging if it's supported. It is called for
MIPS-type processors at CPU-probe stage and attempts to detect whether
the write merging is available. If it's known to be supported and
switchable, then switch on the full mode. Otherwise just perform the
CP0.Config.MM field analysis.

In addition there are platforms like InterAptiv/ProAptiv, which do have
the MM bit field set by default, but having write-through cacheing
unsupported makes write-merging also unsupported. In this case we just
ignore the MM field value.

Co-developed-by: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Signed-off-by: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>