ARM: dts: aspeed: Drop syscon compatible from EDAC in g6 dtsi

ARM: dts: aspeed: Drop syscon compatible from EDAC in g6 dtsi

Its presence is not required by the binding, its addition was not
discussed in commit aac82707fa45 ("ARM: dts: aspeed: Add AST2600 EDAC
into common devicetree"), and its inconsistent with the g4 and g5
dtsis.

Further, the corresponding driver instantiates its own regmap rather
than fetching the syscon regmap, in theory breaking any users of the
syscon, but of which there appear to be none in-tree.

Drop it for now, and we can add it back with the necessary rework if
it's ever required.

Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>