PCI: cadence: Implement capability search using PCI core APIs

The PCI core now provides generic PCI_FIND_NEXT_CAP() and
PCI_FIND_NEXT_EXT_CAP() macros to search for PCI capabilities, using
config accessors we supply.

Use them in the CDNS driver to add cdns_pcie_find_capability() and
cdns_pcie_find_ext_capability() interfaces.

Signed-off-by: Hans Zhang <18255117159@163.com>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250813144529.303548-6-18255117159@163.com