PCI: dwc: Add debugfs based Error Injection support for DWC

Add support to provide Error Injection interface to userspace.

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

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/20250221131548.59616-5-shradha.t@samsung.com


[kwilczynski: commit log, tidy up code comments, update documentation,
change debugfs property name from "duplicate_dllp" to "duplicate_tlp"]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>