ARM: socfpga: dts: add missing clock gates to socfpga.dtsi

The gates for the clocks coming out of the sdram pll
were missing.  The change adds the missing nodes to
the device tree.

Signed-off-by: Matthew Gerlach <mgerlach@opensource.altera.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>