PCI: dwc: Add debugfs based Silicon Debug support for DWC

Add support to provide Silicon Debug 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>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Tested-by: Hrishikesh Deleep <hrishikesh.d@samsung.com>
Link: https://lore.kernel.org/r/20250221131548.59616-4-shradha.t@samsung.com
[kwilczynski: commit log, tidy up Kconfig and drop "default y", tidy up
code comments, squashed patch that fixes a NULL pointer dereference when
debugfs is already unavailable during clean-up from
https://lore.kernel.org/linux-pci/20250225171239.19574-2-manivannan.sadhasivam@linaro.org,
refactor dwc_pcie_debugfs_init() to not return errors, squashed patch that
changes how lack of the RAS DES capability is handled from
https://lore.kernel.org/linux-pci/20250304151814.6xu7cbpwpqrvcad5@thinkpad

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