Microblaze: added defconfig for Avnet S6LX9 Microboard

The Avnet S6LX9 Microboard is a small USB-Stick sized module
containing a Spartan6 FPGA capable of running the Microblaze
softcore processor together with RAM and FLASH memory.

This board support pachage assumes that the Microblaze Bitfile
available from the Avnet website is programmed into the FPGA.

Signed-off-by: Stephan Hoffmann <sho@relinux.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>