PCI/TPH: Add TPH documentation

PCI/TPH: Add TPH documentation

Add a document for the TPH feature, including description of "notph" kernel
parameter and the API interface.

Co-developed-by: Eric Van Tassell <Eric.VanTassell@amd.com>
Link: https://lore.kernel.org/r/20241002165954.128085-4-wei.huang2@amd.com
Signed-off-by: Eric Van Tassell <Eric.VanTassell@amd.com>
Signed-off-by: Wei Huang <wei.huang2@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>