bus: mhi: host: Add mhi_power_down_keep_dev() API to support system suspend/hibernation

bus: mhi: host: Add mhi_power_down_keep_dev() API to support system suspend/hibernation

Currently, ath11k fails to resume from system suspend/hibernation on some
the x86 host machines with below error message:

```
ath11k_pci 0000:06:00.0: timeout while waiting for restart complete
```

This happens because, ath11k powers down the MHI stack during suspend and
that leads to destruction of the struct device associated with the MHI
channels. And during resume, ath11k calls calling mhi_sync_power_up() to
power up the MHI subsystem and that eventually calls the driver framework's
device_add() API from mhi_create_devices(). But the PM framework blocks the
struct device creation during device_add() and this leads to probe deferral
as below:

```
mhi mhi0_IPCR: Driver qcom_mhi_qrtr force probe deferral
```

The reason for deferring device creation during resume is explained in
dpm_prepare():

        /*
         * It is unsafe if probing of devices will happen during suspend or
         * hibernation and system behavior will be unpredictable in this
         * case. So, let's prohibit device's probing here and defer their
         * probes instead. The normal behavior will be restored in
         * dpm_complete().
         */

Due to the device probe deferral, qcom_mhi_qrtr_probe() API is not getting
called during resume and thus MHI channels are not prepared. So this blocks
the QMI messages from being transferred between ath11k and firmware,
resulting in a firmware initialization failure.

After consulting with Rafael, it was decided to not destroy the struct
device for the MHI channels during system suspend/hibernation because the
device is bound to appear again during resume.

So to achieve this, a new API called mhi_power_down_keep_dev() is
introduced for MHI controllers to keep the struct device when required.
This API is similar to the existing mhi_power_down() API, except that it
keeps the struct device associated with MHI channels instead of destroying
them.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03125-QCAHSPSWPL_V1_V2_SILICONZ_LITE-3.6510.30

Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20240305021320.3367-2-quic_bqiang@quicinc.com
[mani: reworded the commit message and subject]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>