PCI: dwc: Add debugfs based Statistical Counter support for DWC

Add support to provide Statistical Counter interface to userspace.

This set of debug registers are part of the RAS DES feature present in
DesignWare PCIe controllers.

Co-developed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Shradha Todi <shradha.t@samsung.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Tested-by: Hrishikesh Deleep <hrishikesh.d@samsung.com>
Link: https://lore.kernel.org/r/20250221131548.59616-6-shradha.t@samsung.com
[kwilczynski: commit log, tidy up code comments, update documentation,
squashed patch that checks if the event counter is supported from
https://lore.kernel.org/linux-pci/20250225171239.19574-3-manivannan.sadhasivam@linaro.org

]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>