bus: mhi: host: pci_generic: Add missing poweroff() PM callback

During hibernation process, once thaw() stage completes, the MHI endpoint
devices will be in M0 state post recovery. After that, the devices will be
powered down so that the system can enter the target sleep state. During
this stage, the PCI core will put the devices in D3hot. But this transition
is allowed by the MHI spec. The devices can only enter D3hot when it is in
M3 state.

So for fixing this issue, let's add the poweroff() callback that will get
executed before putting the system in target sleep state during
hibernation. This callback will power down the device properly so that it
could be restored during restore() or thaw() stage.

Cc: stable@vger.kernel.org
Fixes: 5f0c2ee1 ("bus: mhi: pci-generic: Fix hibernation")
Reported-by: Hemant Kumar <quic_hemantk@quicinc.com>
Suggested-by: Hemant Kumar <quic_hemantk@quicinc.com>
Link: https://lore.kernel.org/r/20220405125907.5644-1-manivannan.sadhasivam@linaro.org


Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>