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>

arch/parisc/include/asm/serial.h

@@ -6,5 +6,3 @@
  * This is used for 16550-compatible UARTs
  */
 #define BASE_BAUD ( 1843200 / 16 )
-
-#define SERIAL_PORT_DFNS