mips: Add CPS_NS16550_WIDTH config

On some platforms IO-memory might require to use a proper load/store
instructions (like Baikal-T1 IO-memory). To fix the cps-vec UART debug
printout let's add the CONFIG_CPS_NS16550_WIDTH config to determine which
instructions lb/sb, lh/sh or lw/sw are required for MMIO operations.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@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>