ath11k: reset MHI during power down and power up

For QCA6390, normal power up and power down can't bring MHI
to a workable state. This happens especially in warm reboot
and rmmod and insmod. Host needs to write a few registers to
bring MHI to normal state.

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: Carl Huang <cjhuang@codeauroro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1597555891-26112-10-git-send-email-kvalo@codeaurora.org