dt-bindings: net: qcom,ipa: make imem interconnect optional

On some newer SoCs, the interconnect between IPA and SoC internal
memory (imem) is not used.  Update the binding to indicate that
having just the memory and config interconnects is another allowed
configuration.

Signed-off-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/20210811141802.2635424-1-elder@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>