soc/cores/spi: Add SPIBone import.

Allow importing SPIBone with:
from litex.soc.cores.spi import SPIBone

litex/soc/cores/spi/__init__.py

@@ -1,3 +1,6 @@
 # SPI-Master/Slave.
 from litex.soc.cores.spi.spi_master import SPIMaster
 from litex.soc.cores.spi.spi_slave import SPISlave
+
+# SPI-Bone.
+from litex.soc.cores.spi.spi_bone import SPIBone