ARM: dts: socfpga: add specific compatible strings for boards

Add a more specific board compatible entry for all of the SOCFPGA
Cyclone 5 based boards.

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
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v3: Be a bit more specific with the c5 dk and sockit, use
    "altr,socfpga-cyclone5-socdk" and "terasic,socfpga-cyclone5-sockit"
v2: remove extra space and add a comma between compatible entries