dt-bindings: memory-controllers: tegra: Fix type references

Json-schema requires a $ref to be under an 'allOf' if there are
additional constraints otherwise the additional constraints are
ignored. (Note that this behavior will be changed in draft8.)

Fixes: 641262f5 ("dt-bindings: memory: Add binding for NVIDIA Tegra30 External Memory Controller")
Fixes: 785685b7 ("dt-bindings: memory: Add binding for NVIDIA Tegra30 Memory Controller")
Fixes: 8da65c37 ("dt-bindings: memory: tegra30: Convert to Tegra124 YAML")
Cc: Thierry Reding <treding@nvidia.com>
Cc: Jonathan Hunter <jonathanh@nvidia.com>
Cc: linux-tegra@vger.kernel.org
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>