PCI: Add new PCI_VPD_RO_KEYWORD_SERIALNO macro

This patch adds a new macro for serial number keyword.

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>