ath11k: Add register access logic for WCN6750

WCN6750 uses static window mapping to access the HW registers.
Unlike QCN9074 which uses 3rd window for UMAC and 2nd window
for CE register access, WCN6750 uses 1st window for UMAC
and 2nd window for CE registers.

Also, refactor the code so that WCN6750 can use the existing
ath11k_pci_read32/write32() APIs for accessing the registers.

Tested-on: WCN6750 hw1.0 AHB WLAN.MSL.1.0.1-00887-QCAMSLSWPLZ-1
Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-01720.1-QCAHSPSWPL_V1_V2_SILICONZ_LITE-1
Tested-on: QCN9074 hw1.0 PCI WLAN.HK.2.5.0.1-01100-QCAHKSWPL_SILICONZ-1
Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.4.0.1-00192-QCAHKSWPL_SILICONZ-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/20220429170502.20080-5-quic_mpubbise@quicinc.com