Replace APB with AXI Sub and remove the need for SOCs to gasket APB<->AXI

[morris-c-mao]

Is it possible to have the interface configurable by parameter or define? Some SoC system bus is AXI; some are AHB or bridging to APB.

[calebofearth]

Is it possible to have the interface configurable by parameter or define? Some SoC system bus is AXI; some are AHB or bridging to APB.

@morris-c-mao there is no current plan to support both modes. Gen2 will only support the AXI interface.