Add a downstream interface feature to the AXI4 adapter

Right now this tool generates an AXI4 peripheral that only gives access to registers and dual-port memories.

Make it possible to connect the AXI4 peripheral generated by this tool with downstream "external" components (which for now are limited to other AXI4 peripherals).

A major rework of the AXI4 adapter was necessary.

This is necessary for:

• modules with their own address map that are sub-modules of this one
• the simple SPI adapter we use to map addresses to registers on e.g. the Analog Devices AD9510