dt-bindings: memory: lpddr3-timings: convert to dtschema

Convert the LPDDR3 memory timings bindings to DT schema format.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220206135807.211767-3-krzysztof.kozlowski@canonical.com