wifi: ath11k: Register shutdown handler for WCN6750

When the system shuts down, SMMU driver will be stopped and
will not assist in IOVA translations. SMMU driver expects all
of its consumers to shutdown before shutting down itself.
WCN6750 being one of the consumer device should not perform any
DMA operations after the SMMU has shutdown which will otherwise
result in SMMU faults.

SMMU driver will call the shutdown() callback of all its
consumer devices and the consumers shall stop further DMA
activity after the invocation of their respective shutdown()
callbacks.

Register the shutdown() callback to the platform core for WCN6750.
Change will not impact other AHB ath11k devices.

Tested-on: WCN6750 hw1.0 AHB WLAN.MSL.1.0.1-00887-QCAMSLSWPLZ-1

Signed-off-by: Manikanta Pubbisetty <quic_mpubbise@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220720134710.15523-1-quic_mpubbise@quicinc.com