PCI: mvebu: Add support for PCI Bridge Subsystem Vendor ID on emulated bridge

Register with Subsystem Device/Vendor ID is at offset 0x2c. Export is via
emulated bridge.

After this change Subsystem ID is visible in lspci output at line:

  Capabilities: [40] Subsystem

Link: https://lore.kernel.org/r/20220222155030.988-7-pali@kernel.org


Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>