clk: rk3399: Set empty for HCLK_SD assigned-clocks

Due to v5.7-rc1 sync the SD controller nodes in rk3399.dtsi
have HCLK_SD assigned-clocks which are usually required for
Linux and don't require to handle them in U-Boot.

 assigned-clocks = <&cru HCLK_SD>;

So, mark them as empty in clock otherwise device probe on
those SD controllers would fail.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>