xilinx_spi: Initial impl. of Xilinx SPI controller

Device model for xilinx XPS SPI controller (v2.0)

Signed-off-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>

hw/microblaze/Makefile.objs

@@ -1,6 +1,7 @@
 obj-y = petalogix_s3adsp1800_mmu.o
 obj-y += petalogix_ml605_mmu.o
 obj-y += microblaze_boot.o
+obj-y += xilinx_spi.o
 
 obj-y += microblaze_pic_cpu.o
 obj-y += xilinx_ethlite.o