clk: rockchip: rk3568: Select the eMMC core clock by rate

[manu]

Summary

The eMMC core clock is a mux without a divider, whose parents provide the
375kHz clock for card identification and the 24MHz to 200MHz clocks for the
other modes. Use RK_CLK_MUX_ROUND_PARENT so that the host driver can change
the card clock, which could not be done before.