dt-bindings: opp: qcom-nvmem: Make speedbin related properties optional

Not all Qualcomm platforms need to care about the speedbin efuse,
nor the value blown into the speedbin efuse.
Therefore, make the nvmem-cells and opp-supported-hw properties
optional.

Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>
Reviewed-by: Ilia Lin <ilia.lin@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>