dt-bindings: misc: qcom,fastrpc: restrict channel names

The Qualcomm FastRPC is expected to in GLINK or SMD edge node and its
channel name is always the same.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220909125403.803158-4-krzysztof.kozlowski@linaro.org


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