ARM: socfpga: dts: Add a clock node for sdmmc CIU

The CIU(Card Interface Unit) get its clock from the sdmmc_clk_divided clock
which is used to clock the card. The sdmmc_clk_divided clock is the sdmmc_clk
passed through a fixed divider of 4. This patch adds the sdmmc_clk_divided
node and makes the sdmmc_clk it's parent.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
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v2: renamed ciu_clk to sdmmc_clk_divided