Fix AXI4 example.
Fix AXI4 example.

I accidentally stumbled into a working AXI4 configuration by multiplying
pbus.beatBytes by 8, but it was fragile. This is the "right way" to add
an AXI4 peripheral.

ucb-bar/chipyard,ucb-bar/chipyard,ucb-bar/chipyard,ucb-bar/chipyard