arch/m68knommu: Removing dead 68328_SERIAL_UART2 config option

CONFIG_68328_SERIAL_UART2 doesn't exist in Kconfig, therefore removing
all references to it from the source.

Signed-off-by: Christian Dietrich <qy03fugy@stud.informatik.uni-erlangen.de>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>