wifi: ath11k: implement SRAM dump debugfs interface

On-board SRAM contains valuable information for firmware debugging so
add a new file named "sram" to debugfs with which we can dump SRAM
content using following the following:

cp /sys/kernel/debug/ath11k/wcn6855\ hw2.0/sram /tmp/sram

Currently this feature is enabled for QCA6390 and WCN6855.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1
Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-01720.1-QCAHSPSWPL_V1_V2_SILICONZ_LITE-1

Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220802075533.1744-3-quic_bqiang@quicinc.com