PCI/VPD: Move VPD sysfs code to vpd.c

Move the VPD-related sysfs code from pci-sysfs.c to vpd.c.  This follows
the pattern of pcie_aspm_create_sysfs_dev_files().  The goal is to
encapsulate all the VPD code and structures in vpd.c.

No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>