mhi: core: add support for priority based event rings

Expand the priority for event rings to allow support for low and
high priority event rings. Low priority events are processed in
a worker thread and high priority events use tasklets scheduled
with high priority.

CRs-Fixed: 2490298
Change-Id: Id9d0a8d3d1e84154643f101e150e1d8c40bb6f94
Acked-by: Bhaumik Vasav Bhatt <bbhatt@qti.qualcomm.com>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>