spi: qcom-geni: Add shutdown and panic notifier support

Praveen Talari <praveen.talari@oss.qualcomm.com> says:

On VM-based platforms, if an SPI DMA transfer is in progress when the
guest is torn down (via reboot/shutdown or a panic/crash), the DMA
engine can keep issuing transactions to IOVAs that have already been
invalidated as part of teardown. The SMMU then raises context faults,
which can affect other VMs sharing the same SMMU instance and obscure
the real root cause of the crash.

This series adds two independent quiesce paths for the GENI SPI
controller so that any in-progress transfer is stopped and the DMA
engine is left idle before the IOVA mappings are torn down:

- Patch 1 adds a platform shutdown() callback that suspends the SPI
  controller (via spi_controller_suspend()) on a normal
  reboot/shutdown path, where sleeping is safe.

- Patch 2 registers a panic notifier that cancels/aborts the
  in-flight command and resets the TX/RX DMA FSMs (or terminates the
  GPI DMA channels) when the kernel panics, covering the crash path
  as well. The notifier bails out early if the device is not
  runtime-active or has no active command, and otherwise uses
  readl_poll_timeout_atomic() to poll status registers directly
  instead of waiting on completions/IRQs like the regular
  error-handling path does, since panic notifiers run with IRQs and
  preemption disabled. The notifier is registered before
  devm_spi_register_controller() so a panic during child device
  probing is still handled.

Link: https://patch.msgid.link/20260818-add-shutdown-and-panic-notifier-for-spi-v3-0-8b62c4bc2d21@oss.qualcomm.com

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