Remove unused CONFIG_SERIAL_SOFTWARE_FIFO feature

This patch removes the completely unused CONFIG_SERIAL_SOFTWARE_FIFO
feature from U-Boot. It has only been implemented for PPC4xx and was not
used at all. So let's remove it and make the code smaller and cleaner.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Detlev Zundel <dzu@denx.de>