dt-bindings: net: qcom,ipa: add optional qcom,qmp property

For some systems, the IPA driver must make a request to ensure that
its registers are retained across power collapse of the IPA hardware.
On such systems, we'll use the existence of the "qcom,qmp" property
as a signal that this request is required.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>