PCI: cadence: Fix updating Vendor ID and Subsystem Vendor ID register

PCI: cadence: Fix updating Vendor ID and Subsystem Vendor ID register

Commit 5b9bd4d8f11c ("PCI: cadence: Add host driver for Cadence PCIe
controller") in order to update Vendor ID, directly wrote to
PCI_VENDOR_ID register. However PCI_VENDOR_ID in root port configuration
space is read-only register and writing to it will have no effect.
Use local management register to configure Vendor ID and Subsystem Vendor
ID.

Link: https://lore.kernel.org/r/20200722110317.4744-10-kishon@ti.com
Fixes: 5b9bd4d8f11c ("PCI: cadence: Add host driver for Cadence PCIe controller")
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>