serwb/efinix: name rx_sys clkout

[rtucker85]

This is useful when trying to use this clock to feed another PLL.

E.g.

        self.sys_pll = sys_pll = TRIONPLL(platform)
        sys_pll.register_clkin(
            clkin = serwb_slave_phy.serdes.clocking.cd_rx_sys.clk,
            freq = sys_clk_freq,
            name = "rx_sys"
        )

[trabucayre]

Applied. Thanks @rtucker85