bus: mhi: pci_generic: Add support for runtime PM

bus: mhi: pci_generic: Add support for runtime PM

When the device is idle it is possible to move it into the lowest MHI
PM state (M3). In that mode, all MHI operations are suspended and the
PCI device can be safely put into PCI D3 state.

The device is then resumed from D3/M3 either because of host initiated
MHI operation (e.g. buffer TX) or because the device (modem) has
triggered wake-up via PME feature (e.g. on incoming data).

Same procedures can be used for system wide or runtime suspend/resume.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/1614971808-22156-6-git-send-email-loic.poulain@linaro.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>