In the asynchronous system, a separate clock is required for the RX and TX
clocks. TXOUTCLK from one transceiver is sent to an MMCM, which
generates the TX userclks for all of the transceivers and for
tx_serdes_refclk in the Interlaken core. RXRECCLK from one
transceiver is sent to an MMCM, which generates the RX userclk for all
of the transceivers and for the rx_serdes_clk in the core.