arm64: dts: imx93: correct SDHC clk entry

DUMMY clk only works with clk_ignore_unused and bootloader enables those
clks that required for SDHC work properly.

Correct SDHC clk entry with real clk.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>