msm: dt-bindings: ipa: add ipa_mpm description

Add ipa_mpm  description to enable MHI PRIME for IPA
to IPA communication for tethering offload.

Change-Id: Ie284437a2cc8d07a27d695d0169944c0090d84c7
Acked-by: Jyothi <jyothij@qti.qualcomm.com>
Signed-off-by: Michael Adisumarta <madisuma@codeaurora.org>