PCI: pci-bridge-emul: Add support for PCI Bridge Subsystem Vendor ID capability

PCI: pci-bridge-emul: Add support for PCI Bridge Subsystem Vendor ID capability

This is read-only capability in PCI config space. Put it between base PCI
capability and base PCI Express capability.

Driver just have to specify subsystem_vendor_id and subsystem_id fields in
emulated bridge structure and pci-bridge-emul takes care of correctly
compose PCI Bridge Subsystem Vendor ID capability.

Link: https://lore.kernel.org/r/20220222155030.988-4-pali@kernel.org
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>