m68k/serial: SERIAL_PORT_DFNS only if CONFIG_ISA

Only define SERIAL_PORT_DFNS when CONFIG_ISA is defined. Otherwise the first
4 slots in the 8250 driver are unavailable on non-ISA machines.

Signed-off-by: Kars de Jong <jongk@linux-m68k.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>