ath11k: register MHI controller device for QCA6390

Modem Host Interface (MHI) is a communication protocol to communicate with
external Qualcomm modems and Wi-Fi chipsets over high speed peripheral buses.
Even though MHI doesn’t dictate underlying physical layer, protocol and MHI
stack is structured for PCI based devices.

Register directly with MHI subsystem as a MHI device driver for firmware
download to QCA6390.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1
Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.1.0.1-01238-QCAHKSWPL_SILICONZ-2

Signed-off-by: Govind Singh <govinds@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1597309466-19688-9-git-send-email-kvalo@codeaurora.org