serial: remove NWP serial support

The NWP serial driver is no longer needed, as the two users of
this hardware have migrated to a much faster generation hardware,
see https://en.wikipedia.org/wiki/QPACE2

 for the replacement.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Benjamin Krill <ben@codiert.org>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>