In a synchronous system, rx_serdes_clk and
tx_serdes_refclk can use the same clock. TXOUTCLK
from one transceiver is sent to a Mixed-Mode Clock Manager (MMCM), which generates the
RX and TX userclks for all of the transceivers and for the
rx_serdes_clk and tx_serdes_refclk in the Interlaken 600G core.