Add support for AXI4 and AXI4-Lite type busses

This commit introduces convertion between a foreign bus architecture and
Wishbone. By specifying "bustype=axi4" or "bustype=axi4-lite" on either
a slave or a master the interconnect will embed convertion cores and
change the output wires to be those of AXI4 / AXI4-Lite.

This is compatible with the "datawidth" directive.