PCI/DPC: Expose dpc_process_error(), dpc_reset_link() for use by EDR

If firmware controls DPC, it is generally responsible for managing the DPC
capability and events, and the OS should not access the DPC capability.

However, if firmware controls DPC and both the OS and the platform support
Error Disconnect Recover (EDR) notifications, the OS EDR notify handler is
responsible for recovery, and the notify handler may read/write the DPC
capability until it clears the DPC Trigger Status bit.  See [1], sec 4.5.1,
table 4-6.

Expose some DPC error handling functions so they can be used by the EDR
notify handler.

[1] Downstream Port Containment Related Enhancements ECN, Jan 28, 2019,
    affecting PCI Firmware Specification, Rev. 3.2
    https://members.pcisig.com/wg/PCI-SIG/document/12888

Link: https://lore.kernel.org/r/e9000bb15b3a4293e81d98bb29ead7c84a6393c9.1585000084.git.sathyanarayanan.kuppuswamy@linux.intel.com
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>