ARM: dts: s3c64xx: Remove skeleton.dtsi and fix DTC warnings for /memory

Remove the usage of skeleton.dtsi to fix the DTC warnings:

    arch/arm/boot/dts/s3c6410-mini6410.dtb: Warning (unit_address_vs_reg):
        /memory: node has a reg or ranges property, but no unit name
    arch/arm/boot/dts/s3c6410-smdk6410.dtb: Warning (unit_address_vs_reg):
        /memory: node has a reg or ranges property, but no unit name

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>