wifi: ath12k: Move Wi-Fi 7 MHI configuration to dedicated file

wifi: ath12k: Move Wi-Fi 7 MHI configuration to dedicated file

Relocate target-specific MHI configuration to a new mhi_wifi7.c file to
isolate Wi-Fi 7 related logic from the common codebase.
Improve modularity by separating hardware-dependent code from shared
components.
Enhance maintainability and prepare the driver for clean integration of
additional device families.

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.3.1-00173-QCAHKSWPL_SILICONZ-1
Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3

Signed-off-by: Kiran Venkatappa <quic_kiranv@quicinc.com>
Reviewed-by: Vasanthakumar Thiagarajan <vasanthakumar.thiagarajan@oss.qualcomm.com>
Reviewed-by: Baochen Qiang <baochen.qiang@oss.qualcomm.com>
Link: https://patch.msgid.link/20250812-ath12k-mod-v1-4-8c9b0eb9335d@quicinc.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>