cnss2: Add changes to support MHI bus

Add support to use MHI bus for WLAN QCA6290 chipset. MHI is on top
of PCIe bus and mainly used for downloading firmware and transferring
certain data between host and device.

Change-Id: I5b6993ff120a1568047b288edfb2414e92392fe9
Signed-off-by: Yue Ma <yuem@codeaurora.org>