PCI: dwc: Add helper to find the Vendor Specific Extended Capability (VSEC)

PCI: dwc: Add helper to find the Vendor Specific Extended Capability (VSEC)

The new dw_pcie_find_vsec_capability() helper will be used within
different DWC APIs to find the VSEC capabilities like PTM, RAS, etc.

Co-developed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Shradha Todi <shradha.t@samsung.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Tested-by: Hrishikesh Deleep <hrishikesh.d@samsung.com>
Link: https://lore.kernel.org/r/20250221131548.59616-3-shradha.t@samsung.com
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>