The RGMII speed mux selects between the 125MHz GMAC clock and fixed
dividers of it that provide the 25MHz (100M) and 2.5MHz (10M) clocks.
Without RK_CLK_MUX_REPARENT the mux does not try its parents and the
rate request is forwarded to clk_gmac0 (and above), where it fails with
EBUSY because that clock is enabled by more than one consumer.
Mark both RGMII speed muxes as reparenting so that the rate is achieved
by selecting the appropriate fixed divider.