arm: dts: mt7623: fix invalid memory node being generated

Below two wrong nodes in existing DTS files would cause a fail boot since
in fact the address 0 is not the correct place the memory device locates
at.

memory {
        device_type = "memory";
        reg = <0x0 0x0 0x0 0x0>;
};

memory@80000000 {
        reg = <0x0 0x80000000 0x0 0x40000000>;
};

In order to avoid having a memory node starting at address 0, we can't
include file skeleton64.dtsi and instead need to explicitly manually
define a few of properties the DTS relies on such as #address-cells
and #size-cells in root node and device_type in the node memory@80000000.

Cc: stable@vger.kernel.org
Fixes: 31ac0d69 ("ARM: dts: mediatek: add MT7623 basic support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>