dt-bindings: nvmem: qcom,spmi-sdam: Drop child node schema

Drop the child node schema. The schema for child nodes is already
defined by nvmem.yaml and doesn't need to be duplicated in the
qcom,spmi-sdam schema.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220119151135.3598392-1-robh@kernel.org