Rework the arbiter to not change requests once made

[fabianschuiki]

Currently the way the arbiter is organized can cause the payload of AXI beats (address, data, etc.) to change after the valid signals has been asserted. This violates the AXI4 standard. Rework the arbiter such that it holds on to its decision until the request is cleared. This should be easily doable by having the round-robin flag jump to the selected master.

[andreaskurth]

Until this is fixed, a workaround is to put an AXI Slice between each master port of the interconnect and the slave device.