dt-bindings: arm: altera: Drop socfpga-sdram-edac.txt

The contents of arm/altera/socfpga-sdram-edac.txt are already covered by
edac/altr,socfpga-ecc-manager.yaml except for the "altr,sdram-edac"
compatible string. Add the compatible and drop the old .txt binding doc.

Acked-by: Matthew Gerlach <matthew.gerlach@altera.com>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>