The #interconnect-cells properties are required to hook up memory
clients to the MC/EMC in interconnects properties. Add a description for
these properties.

For the nested EMC controller, the list of required properties was
missing. Add it so that the validation can be more strict.

Also, allow multiple reg entries required by Tegra194 and later.

While at it, also remove the dummy BPMP node from the example because it
is incomplete and fails validation. It's also not necessary for this
file and the BPMP DT schema already has a full example.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>