ARM: tegra: Fix unit_address_vs_reg DTC warnings for /memory

Add a generic /memory node in each Tegra DTSI (with empty reg property,
to be overidden by each DTS) and set proper unit address for /memory
nodes to fix the DTC warnings:

    arch/arm/boot/dts/tegra20-harmony.dtb: Warning (unit_address_vs_reg):
        /memory: node has a reg or ranges property, but no unit name

The DTB after the change is the same as before except adding
unit-address to /memory node.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>