liteeth/gen: use add_csr to fix the order of banks

[mkj]

Device drivers (Linux etc) expect the banks to be at constant locations. We fix them to the locations previously used for microwatt.dts, somewhat arbitrarily.

or1klitex.dts is the other upstream kernel devicetree using liteeth, it may need updating to these fixed addresses (it may not be consistent with current liteeth anyway)