serial: Add driver for the Cell Network Processor serial port NWP device

Add support for the nwp serial device which is connected to a DCR bus. It
uses the of_serial device driver to determine necessary properties from
the device tree.  The supported device is added as serial port number 85.

NWP stands for network processor and it is part of the QPACE - Quantum
Chromodynamics Parallel Computing on the Cell Broadband Engine project.
The implementation is a lightweight uart implementation with the focus
to consume as little resources as possible and it is connected to a
DCR bus.

Signed-off-by: Benjamin Krill <ben@codiert.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>