msm: mhi_dev: Cache event ring read pointer value

Maintain a copy of the event ring read pointer in DMA coherent
memory before scheduling asynchronous DMA to update the host.
This is needed to ensure the correct read pointer value is updated
to host (for which the event DMA was done). Also schedule MSI via
IPA/EDMA to avoid potential early delivery to host (before the event
itself) when it is triggered via the PCIE controller.

Change-Id: I4fdd5b8802677b434a0c2a12118065f264802442
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>