parisc: remove empty SERIAL_PORT_DFNS in serial.h

If architectures don't support SERIAL_PORT_DFNS, they need not define it
to "nothing", the related drivers need do it by themselves (e.g.  8250
serial driver).

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Helge Deller <deller@gmx.de>