ath11k: implement suspend for QCA6390 PCI devices

Now that all the needed pieces are in place implement suspend support QCA6390
PCI devices. All other devices will return -EOPNOTSUPP during suspend. The
suspend is implemented by switching the firmware to WoW mode during suspend, so
the firmware will be running on low power mode while host is in suspend.

At the moment we are not able to shutdown and fully power off the device due to
bugs in MHI subsystem, so WoW mode is a workaround for the time being.

During suspend we enable WoW mode, disable CE irq and DP irq, then put MHI to
suspend state.  During resume, driver resumes MHI firstly, then enables CE irq
and dp IRQ, and sends WoW wakeup command to firmware.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1

Signed-off-by: Carl Huang <cjhuang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1607708150-21066-11-git-send-email-kvalo@codeaurora.org