mhi_dev: mhi: Update MHI interrupt registration

MDM9640 has a dedicated interrupt for MHI A7 which
is used to process doorbells such as processing
channel, event and command configuration. On MDM9650
the MHI A7 interrupt is part of EP PCIe. MHI A7 driver
registers a callback with the EP PCIe driver for
processing doorbells.

Change-Id: Ibd2fbab015ef7b5790e36fb97b415c1c03d80b09
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>