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.  Reflect this in the binding by moving
the definition of the "imem" interconnect to the end and defining
minItems to be 2 for both the interconnects and interconnect-names
properties.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>