dt-bindings: soc: qcom: smd: restrict child name to smd-edge

Child node of SMD must be either smd-edge or rpm, so the pattern can be
narrowed.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220909125403.803158-7-krzysztof.kozlowski@linaro.org


Signed-off-by: Rob Herring <robh@kernel.org>