Rename arbiter -> wb_port_arbiter

In order to avoid name conflicts with other core, change
"arbiter" to a less generic "wb_port_arbiter".

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Use localparam to avoid warnings

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>